Empty beer jug? Guinness, maybe, required (6)
I believe the answer is:
brewer
'guinness maybe required' is the definition.
(Guinness is a brewery)
'empty beer jug?' is the wordplay.
'empty' suggests removing the centre.
'jug?' becomes 'ewer' (type of vessel).
'beer' with its centre taken out is 'br'.
'br'+'ewer'='BREWER'
(Other definitions for brewer that I've seen before include "Reference" , "Ale manufacturer" , "Bitter maker?" , "Maker of 22 Across" , "I make tea" .)