Fancy I must open large barrel, wanting gallon (4)
I believe the answer is:
like
'fancy i' is the definition.
'like' can be an answer for 'fancy' (both can mean to find someone attractive). I am not sure about the 'i' bit.
'must open large barrel wanting gallon' is the wordplay.
'must open large' becomes 'li' (I can't explain this - if you can you should believe this answer much more).
'barrel' becomes 'keg' (keg is a kind of barrel).
'wanting' indicates named letters should be taken away (to want something can mean to lack it or need it).
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'keg' with 'g' taken away is 'ke'.
'li'+'ke'='LIKE'
(Other definitions for like that I've seen before include "Such as; find agreeable" , "Approve; be fond of" , "Relish" , "Find appealing" , "Fancy - resembling" .)