Attacked when returning the keys (3,2)
I believe the answer is:
set on
'attacked' is the definition.
(set on can mean to attack)
'when returning the keys' is the wordplay.
I cannot really see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
The remaining letters 'se' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for set on that I've seen before include "Go" , "Determined" , "Attack" .)