Oppose a law for the benefit of the shop assistant? (10)
I believe the answer is:
counteract
'oppose' is the definition.
(I've seen this before)
'a law for the benefit of the shop assistant?' is the wordplay.
'a law' becomes 'act' (I've seen this before).
'for' means one lot of letters go next to another (I've seen this in other clues).
'the benefit of the shop assistant?' becomes 'counter' (I can't justify this - if you can you should give a lot more credence to this answer).
'act' put after 'counter' is 'COUNTERACT'.
Can you help me to learn more?
(Other definitions for counteract that I've seen before include "Put a check on" , "Nullify" , "Oppose" , "Work against" , "Tell against (something)" .)