Fitting uniform, it's in black (8)
I believe the answer is:
suitable
'fitting' is the definition.
('suitable' can be a synonym of 'fitting')
'uniform it's in black' is the wordplay.
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'black' becomes 'sable' (sable is a kind of black**).
'u'+'it'='uit'
'uit' going inside 'sable' is 'SUITABLE'.
(Other definitions for suitable that I've seen before include "Becoming" , "Appropriate, befitting" , "Appropriate, apt" , "qualified" , "In character" .)