Be almost certain it's unfermented wine (4)
I believe the answer is:
must
'unfermented wine' is the definition.
(I've seen this before)
'be almost certain it's' is the wordplay.
'be almost certain' becomes 'mus' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'mus'+'t'='MUST'
Can you help me to learn more?
(Other definitions for must that I've seen before include "Prerequisite" , "new wine" , "Unfermented wine" , "The mould is compelled" , "Smell of dampness or staleness" .)