Taking this to repair appliance perhaps retreat under challenge (4,3)
I believe the answer is:
back off
'retreat under challenge' is the definition.
'back off' can be an answer for 'retreat' (backing off is a kind of retreating). I am not sure about the 'under challenge' bit.
'taking this to repair appliance perhaps' is the wordplay.
I cannot quite understand how this works, but
'perhaps' could be 'back' (I've seen this in other clues) and 'back' is present in the answer.
The remaining letters 'off' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for back off that I've seen before include "Retreat" , "Give up" , "Withdraw from confrontation" , "as TV may have to be repaired" .)