I'm in prison without money, that's the snag (10)
I believe the answer is:
'the snag' is the definition.
'i'm in prison without money' is the wordplay.
'in prison' becomes 'pent' ('pent' can mean 'confined').
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'money' becomes 'dime'.
'pent' placed around 'dime' is 'pediment'.
'that's' is the link.
(Other definitions for impediment that I've seen before include "Hindrance, obstruction" , "opposition" , "Let" , "Obstacle" .)