Number one drink in West? (7)
I believe the answer is:
imagine
'number' is the definition.
(I've seen this before)
'one drink in west?' is the wordplay.
'one' becomes 'I' (Roman numeral).
'drink' becomes 'gin' (gin is a kind of drink).
'in' is an insertion indicator.
'west?' becomes 'mae' (actress Mae West).
'gin' placed within 'mae' is 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "Picture mentally" , "Conceive; fancy" , "Number one" , "Believe to be so, suppose" , "Picture in one's mind" .)