A policeman when not on duty is unconventional (7)
I believe the answer is:
'unconventional' is the definition.
(similar in meaning)
'a policeman when not on duty' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for offbeat that I've seen before include "Quirky" , "Unconventional or lacking rhythm" , "Kooky" , "Odd, unconventional" , "Strange" .)