Giving drug to pet? What's wrong in that? (6)
I believe the answer is:
dosing
'giving drug to' is the definition.
(giving a dosage of a drug)
'pet? what's wrong in that?' is the wordplay.
'pet?' becomes 'dog'.
'what's wrong' becomes 'sin' (something morally wrong).
'in that?' means one lot of letters goes inside another.
'dog' going around 'sin' is 'DOSING'.
(Other definitions for dosing that I've seen before include "Dispensing" , "Giving medicine" .)