Make worse when half of them have beer (8)
I believe the answer is:
embitter
'make worse' is the definition.
(thesaurus)
'half of them have beer' is the wordplay.
'half of' suggests halving the letters.
'have' means one lot of letters go next to another.
'beer' becomes 'bitter' (I've seen this before).
'them' cut in half is 'em'.
'em'+'bitter'='EMBITTER'
'when' is the link.
(Other definitions for embitter that I've seen before include "Create resentment" , "Make resentful and hostile" , "poison" , "Will exasperate" , "Alienate" .)