It is used for lubricating locks (4,3)
I believe the answer is:
hair oil
'it is used for lubricating' is the definition.
The definition suggests a singular noun which matches the answer.
'locks' is the wordplay.
I cannot really understand how this works, but
'locks' could be 'hair' (locks can mean hair) and 'hair' is found in the answer.
The remaining letters 'oil' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for hair oil that I've seen before include "Brilliantine, for example" , "Toiletry used on the head" .)