Muscle's instant power succeeded (7)
I believe the answer is:
triceps
'muscle's' is the definition.
(I know that triceps is a muscles)
'instant power succeeded' is the wordplay.
'instant' becomes 'trice' (both can mean a short amount of time).
'power' becomes 'p' (symbol in physics).
'succeeded' becomes 's' (genealogical abbreviation).
'trice'+'p'+'s'='TRICEPS'
(Other definitions for triceps that I've seen before include "Muscle that extends the forearm" , "Muscle at the back of the upper arm" , "Upper-arm muscle" , "army feature?" , "Muscles" .)