Arthur's head is in a drink once more (5)
I believe the answer is:
again
'once more' is the definition.
('again' can be a synonym of 'once more')
'arthur's head is in a drink' is the wordplay.
'head' indicates taking the first letters.
'is in' indicates putting letters inside.
'drink' becomes 'gin' (gin is a kind of drink**).
The initial letter of 'arthurs' is 'a'.
'a'+'gin'='agin'
'a' put into 'agin' is 'AGAIN'.
(Other definitions for again that I've seen before include "Moreover - a second time" , "On the other hand" , "10 10 [AGAIN" , "Anew" , "'Another time, once more (5)'" .)