Soldier on, bad time to retreat (7)
I believe the answer is:
trooper
'soldier' is the definition.
(trooper is a kind of soldier)
'on bad time to retreat' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'bad' becomes 'poor' (similar in meaning).
'time' becomes 't' (abbreviation).
'to retreat' says the letters should be written backwards.
're'+'poor'+'t'='repoort'
'repoort' back-to-front is 'TROOPER'.
(Other definitions for trooper that I've seen before include "Cavalry 21" , "He swears" , "Soldier, proverbial swearer" , "Cavalry soldier" , "Private soldier" .)