Something additional after the blink of an eye muscle (7)
I believe the answer is:
triceps
'muscle' is the definition.
(triceps is a kind of muscle)
'something additional after the blink of an eye' is the wordplay.
'something additional' becomes 'ps' (postscript in a letter).
'after' means one lot of letters go next to another.
'the blink of an eye' becomes 'trice' (both can mean a very short amount of time).
'ps' after 'trice' is 'TRICEPS'.
(Other definitions for triceps that I've seen before include "Source of uplifting strength" , "One stretches out an arm!" , "Muscles" , "Muscle at the back of the upper arm" , "army feature?" .)