Text added after very short time shows muscle (7)
I believe the answer is:
triceps
'muscle' is the definition.
(I know that triceps is a muscle)
'text added after very short time' is the wordplay.
I cannot quite see how this works, but
'time' could be 'trice' (I have seen 'Short time ' mean 'trice' so perhaps 'time' could also mean 'trice') and 'trice' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for triceps that I've seen before include "Muscles" , "Extensor muscle" , "Upper-arm muscle" , "army feature?" , "Muscle that extends the forearm" .)