Expire in a uniform: farewell (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'expire in a uniform' is the wordplay.
'expire' becomes 'die' (synonyms).
'in' means one lot of letters goes inside another.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'a'+'u'='au'
'die' inserted inside 'au' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "12 Down" , "Goodbye to the French" , "See you later" , "Comment on leaving" , "All the best" .)