Manner adopted by officer backing court-martial? (5)
I believe the answer is:
trial
'court-martial?' is the definition.
(court-martial is a kind of trial)
'manner adopted by officer backing' is the wordplay.
'manner' becomes 'air' (air can mean someone's manner or character).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'officer' becomes 'lt' (abbreviation for lieutenant).
'backing' says the letters should be written in reverse.
'air' placed into 'lt' is 'lairt'.
'lairt' reversed gives 'TRIAL'.
(Other definitions for trial that I've seen before include "Court appearance on charge - it's an ordeal" , "competition" , "Pilot" , "Ordeal or court case" , "show at Crufts?" .)