Instant and extremely prodigious muscles (7)
I believe the answer is:
triceps
'muscles' is the definition.
(tricep is a type of muscle)
'instant and extremely prodigious' is the wordplay.
'instant' becomes 'trice' (both can mean a short amount of time).
'and' means one lot of letters go next to another.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'prodigious' with its middle taken out is 'ps'.
'trice'+'ps'='TRICEPS'
(Other definitions for triceps that I've seen before include "Muscle at the back of the upper arm" , "Muscles" , "Extensor muscle" , "one can't bend the elbow!" , "Three-headed muscle" .)