Unconventional policeman when not on duty (7)
I believe the answer is:
offbeat
'unconventional' is the definition.
(similar in meaning)
'not on duty' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'duty' becomes 'beat' (I can't explain this - if you can you should give a lot more credence to this answer).
'off'+'beat'='OFFBEAT'
'policeman when' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for offbeat that I've seen before include "Weird - irregular" , "Wacky; syncopated" , "Way-out" , "Quirky" , "rather unorthodox" .)