Top dressing (4,3)
I believe the answer is:
hair oil
'dressing' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'top' is the wordplay.
I cannot quite see how this works, but
'top' could be 'ai' (I've seen this in other clues) and 'ai' is present in the answer.
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" .)