Fat NCO advanced without uniform (9)
I believe the answer is:
corpulent
'fat' is the definition.
(similar in meaning)
'nco advanced without uniform' is the wordplay.
'nco' becomes 'corp' (I've seen this in other clues).
'advanced' becomes 'lent' (advance can mean to lend).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'corp'+'lent'='corplent'
'corplent' placed around 'u' is 'CORPULENT'.
(Other definitions for corpulent that I've seen before include "Overweight" , "Having a bulky body, fat" , "Rotund, obese" , "Plump" , "Portly" .)