Expire wearing a uniform? Farewell (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'expire wearing a uniform?' is the wordplay.
'expire' becomes 'die' (synonyms).
'wearing' indicates putting letters inside.
'uniform?' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'a'+'u'='au'
'die' placed within 'au' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "Farewel" , "Farewell, Monsieur!" , "Some say it when leaving" , "Bye" , "Goodbye, mademoiselle" .)