Persian utterance lost after battle — that's painful (5)
I believe the answer is:
miaow
'persian utterance' is the definition.
(utterance a Persian cat might make)
'lost after battle that's painful' is the wordplay.
'lost after battle' becomes 'MIA' (missing in action).
'that's painful' becomes 'ow' (I've seen this in other clues).
'mia'+'ow'='MIAOW'
(Other definitions for miaow that I've seen before include "Beastly remark" , "Cat-call" , "that the Queen's going?" , "Cat call?" , "catty remark" .)