The following excerpt is from The Guardian:
She needs to prove that she isn’t still in the hands of big money. She has only been proving the opposite again and again. I want to vote for Hillary. I just need a reason to do so.
Please explain the meaning of the phrase in bold. I can only guess it may mean that somehow she was involved in some fraudulent financial activities in the past and now she needs to prove that she's no longer, but I'm not sure.
The American Heritage® Dictionary of Idioms defines:
in the hands of: In the possession of; in the custody or under the authority of.
If we consider that collective mentality usually assumes that big money can only be made unorthodoxly then my guess could probably be true. On the other hand, being in the hands of big money might mean that she's controlled by big money, that is, wealthy people, but these are just my thoughts, so please don't get me wrong.
Best Answer
Pretty much this is what it means.
Although it might not mean wealthy people specifically but entities that have a lot of money to contribute to campaign funds or lobby. These could include political action committees of both individuals, corporations (in the US corporations are legally equivalent to people in a lot of situations), sets of individuals, and industry alliances (e.g. RIAA, MPAA, etc.)
Similar terms are "Big Business", "Big Oil",
and a new one "Big Data"to mean the extreme power - political, financial or otherwise - wielded by the powerful members of a certain class.