Shirt’s back occupied by most of succinct descriptive comment (7)
I believe the answer is:
epithet
'descriptive comment' is the definition.
'shirt's back occupied by most of succinct' is the wordplay.
'shirt' becomes 'tee' (tee shirt is a kind of shirt).
'back' shows that the letters should be reversed in order.
'occupied by' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'succinct' becomes 'pithy' (I've seen this before).
'tee' back-to-front is 'eet'.
'pithy' with its final letter taken off is 'pith'.
'eet' placed around 'pith' is 'EPITHET'.
(Other definitions for epithet that I've seen before include "Fitting description" , "Descriptive term or short phrase" , "Descriptive word or phrase added to a name" , "Descriptive adjective" , "Adjective expressing some quality of a person" .)