Text added after very short time shows muscle (7)
I believe the answer is:
triceps
'muscle' is the definition.
(triceps is a kind of muscle)
'text added after very short time' is the wordplay.
I cannot really 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 within the answer.
This explanation may well be incorrect...
'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" .)