Uniform used in particular Jewish festival (5)
I believe the answer is:
purim
'jewish festival' is the definition.
(I've seen this in another clue)
'uniform used in particular' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'used in' is an insertion indicator.
'particular' becomes 'prim' (prim can mean precise or particular).
'u' put within 'prim' is 'PURIM'.
(Other definitions for purim that I've seen before include "Jewish holy day" , "Jewish feast" , "Jewish festival" .)