It just has to be used in winemaking (4)
I believe the answer is:
must
'has to be used in winemaking' is the definition.
I can't tell whether this definition defines the answer.
'it' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'mus' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'just' is the link.
Can you help me to learn more?
(Other definitions for must that I've seen before include "Make off with (plane)" , "Smell of dampness or staleness" , "Essential to have" , "new wine" , "Mouldiness" .)