Mark brews soundly (6)
I believe the answer is:
bruise
'mark' is the definition.
(I have seen 'Mark of blow ' mean 'bruise' so perhaps 'mark' could also mean 'bruise')
'brews soundly' is the wordplay.
'soundly' shows a homophone (sound like).
'brews' sounds like 'BRUISE'.
(Other definitions for bruise that I've seen before include "Mark left by blow" , "Rubies (anag.)" , "Injury mark; sounds like ferments" , "Discoloured injury" , "Discolour" .)