Hunter about to imbibe port (5)
I believe the answer is:
orion
'hunter' is the definition.
(constellation)
'about to imbibe port' is the wordplay.
'about' becomes 'on' (a book about x is on x).
'to imbibe' means one lot of letters goes inside another (to imbibe can mean to drink or absorb).
'port' becomes 'rio' (Rio de Janeiro is a port).
'on' placed around 'rio' is 'ORION'.
(Other definitions for orion that I've seen before include "Greek hunter of myth or bright constellation" , "Greek hunter became a constellation" , "Prominent constellation" , "Mythical great hunter" , "Heavenly figure" .)