All in one mixing fatty substance (7)
I believe the answer is:
lanolin
'all in' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'one mixing fatty substance' is the wordplay.
I cannot really see how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'substance' could be 'li' (Li is an example) and 'li' is found within 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 lanolin that I've seen before include "Ointment containing wool fat" , "Oil extracted from wool" , "substance found in ointments" , "Oily substance on wool" , "Wool extract used in some ointments" .)