Second-hand uniform has sleeve mended at last (4)
I believe the answer is:
used
'second-hand' is the definition.
(I've seen this before)
'uniform has sleeve mended at last' is the wordplay.
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'at last' suggests the final letters.
The final letters of 'has sleeve mended' are 'sed'.
'u'+'sed'='USED'
(Other definitions for used that I've seen before include "Utilised" , "Employed in task, or second-hand" , "Took advantage of" , "Dog-eared" , "Second-hand - not wasted" .)