Offensive back on standby ending a record (5)
I believe the answer is:
diary
'a record' is the definition.
(I've seen this in another clue)
'offensive back on standby ending' is the wordplay.
'offensive' becomes 'raid' (a raid is a military offensive).
'back' says the letters should be written backwards.
'on' is a charade indicator (letters next to each other).
'ending' indicates one should take the final letters (the letter at the end of the word).
The final letter of 'standby' is 'y'.
'raid' back-to-front is 'diar'.
'diar'+'y'='DIARY'
(Other definitions for diary that I've seen before include "Appointment book" , "Personal record book" , "Personal journal" , "Pepys's record" , "Log of daily activity" .)