Muscle comes in instant, leading to postscript (7)
I believe the answer is:
triceps
'muscle' is the definition.
(I know that triceps is a muscle)
'in instant leading to postscript' is the wordplay.
'in instant' becomes 'trice' (both can mean a short amount of time. I am not sure about the 'in' bit.).
'leading to' says to put letters next to each other (some letters go before or lead others).
'postscript' becomes 'ps' (abbreviation).
'trice'+'ps'='TRICEPS'
'comes' is the link.
(Other definitions for triceps that I've seen before include "Extensor muscle" , "Green citrus fruit" , "army feature?" , "Muscles" , "Muscle at the back of the upper arm" .)