An afternoon brew, when chap's opening the drink down under (6,3)
I believe the answer is:
tasman sea
'drink down under' is the definition.
('the drink' can informally mean the sea)
'an afternoon brew when chap's opening' is the wordplay.
'an afternoon brew' becomes 'tea' (afternoon cup of tea).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'chap's' becomes 'man's' (chap can mean a man).
'opening' is an insertion indicator (some letters open a gap in others).
'as'+'mans'='asmans'
'tea' placed around 'asmans' is 'TASMAN SEA'.
'the' is the link.
(Other definitions for tasman sea that I've seen before include "Ocean area between Australia and New Zealand" , "The main" , "part of Pacific" , "it's wet" , "large quantity of water" .)