Flash Police Sergeant displaying muscle (7)
I believe the answer is:
triceps
'muscle' is the definition.
(triceps is a kind of muscle)
'flash police sergeant' is the wordplay.
'flash' becomes 'trice' (quick moment).
'police sergeant' becomes 'PS'.
'trice'+'ps'='TRICEPS'
'displaying' is the link.
(Other definitions for triceps that I've seen before include "Muscle at the back of the upper arm" , "Extensor muscle" , "Muscles" , "one can't bend the elbow!" , "One stretches out an arm!" .)