Improve gag about old uniform (7)
I believe the answer is:
retouch
'improve' is the definition.
(thesaurus)
'gag about old uniform' is the wordplay.
'gag' becomes 'retch' (I've seen this before).
'about' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'o'+'u'='ou'
'retch' placed around 'ou' is 'RETOUCH'.
(Other definitions for retouch that I've seen before include "Correct painting" , "Fine-tune" , "Make slight improvements to (a photo/picture)" , "Paint over" , "Improve a photo, with a pencil say" .)