Flash skin of pectorals showing muscles (7)
I believe the answer is:
triceps
'muscles' is the definition.
(tricep is a type of muscle)
'flash skin of pectorals' is the wordplay.
'flash' becomes 'trice' (quick moment).
'skin of' says to hollow out the word (remove centre letters).
'pectorals' with its middle removed is 'ps'.
'trice'+'ps'='TRICEPS'
'showing' acts as a link.
(Other definitions for triceps that I've seen before include "army feature?" , "Source of uplifting strength" , "one can't bend the elbow!" , "Muscle at the back of the upper arm" , "Upper-arm muscle" .)