Part of horse’s behind associated with power (7)
I believe the answer is:
pastern
'part of horse's' is the definition.
(part of a horse's leg)
'behind associated with power' is the wordplay.
'behind' becomes 'astern' (at the back or behind of a ship).
'associated with' is a charade indicator (letters next to each other).
'power' becomes 'p'.
'astern' put after 'p' is 'PASTERN'.
(Other definitions for pastern that I've seen before include "Wrongly-defined part of horse" , "Part of horse's foot" , "part of Arab's leg" , "Fetter-bone of horse" , "'The knee of an horse' (Johnson)" .)