Drink absinthe at first in sea (4)
I believe the answer is:
mead
'drink' is the definition.
(mead is a kind of drink)
'absinthe at first in sea' is the wordplay.
'at first' says to take the initial letters.
'in' is an insertion indicator.
'sea' becomes 'med' (abbreviation of Mediterranean).
The initial letter of 'absinthe' is 'a'.
'a' placed into 'med' is 'MEAD'.
(Other definitions for mead that I've seen before include "Cornish drink" , "Honey drink; lea" , "The poetic field" , "Honey-based alcoholic drink" , "Drink made of fermented honey and water" .)