Offensive film location (5)

I believe the answer is:

onset

I'm a little stuck... Click here to teach me more about this clue!

'offensive' is the definition.
(thesaurus)

'film location' is the wordplay.
'film' becomes 'on' (I can't explain this - if you can you should give a lot more credence to this answer).
'location' becomes 'set' (set is a kind of location).
'on'+'set'='ONSET'

Can you help me to learn more?

(Other definitions for onset that I've seen before include "Beginning; military attack" , "Advent" , "Start - notes (anag)" , "Storming" , "Cast stone in attack" .)

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app