Muscles in short time before additional message (7)
I believe the answer is:
triceps
'muscles' is the definition.
(I've seen this before)
'short time before additional message' is the wordplay.
'short time' becomes 'trice' (I have seen 'Short time ' mean 'trice' so perhaps 'time' could also mean 'trice'; 'short' indicates an abbreviation).
'before' says to put letters next to each other.
'additional message' becomes 'ps' (a PS or postscript in a letter).
'trice'+'ps'='TRICEPS'
'in' acts as a link.
(Other definitions for triceps that I've seen before include "Three-headed muscle" , "Source of uplifting strength" , "Muscle that extends the forearm" , "Upper-arm muscle" , "Muscles" .)