The suggestion spell description says the following:
You suggest a course of activity (limited to a sentence or two) and
magically influence a creature you can see within range that can hear
and understand you. Creatures that can't be charmed are immune to this
effect. The suggestion must be worded in such a manner as to make the
course of action sound reasonable. Asking the creature to stab itself,
throw itself onto a spear, immolate itself, or do some other obviously
harmful act ends the spell.The target must make a Wisdom saving throw. On a failed save, it
pursues the course of action you described to the best of its ability.
The suggested course of action can continue for the entire duration.
If the suggested activity can be completed in a shorter time, the
spell ends when the subject finishes what it was asked to do.You can also specify conditions that will trigger a special activity
during the duration. For example, you might suggest that a knight give
her warhorse to the first beggar she meets. If the condition isn't met
before the spell expires, the activity isn't performed.If you or any of your companions damage the target, the spell ends.
Can I use this spell to force a NPC to tell the truth? For example, with the sentence: "For the next hour, you will answer my questions honestly." I don't know if I can consider this a course of action.
Best Answer
My guess is that merely telling truth might work, as the suggestion could nudge the NPC into Bond Villain mode:
"these people are no threat, so I will reveal to them my great plan!"
"I can totally trust these people with this secret that I have been harbouring all these years!"
It does not ensure that the NPC will actually tell the truth, now. If, say, an innocent was Suggested that he reveal his plans when he actually has none, he might spin some up just to please the suggestion.
Your given wording, though, is not a good suggestion. A better one might be something like "You want to show us how dumb we are and reveal your real plans." Or "You absolutely trust me and will tell me everything you know." Your DM may actually like it, as it gives him a chance to roleplay a monologue.