Extra time I'd serve up in a disreputable bar (8)
I believe the answer is:
additive
'extra' is the definition.
'time i'd serve up in a disreputable bar' is the wordplay.
'time' becomes 't'.
'serve up' says the letters should be written in reverse.
'in' indicates putting letters inside.
'disreputable bar' becomes 'dive'.
't'+'id'='tid'
'tid' back-to-front is 'dit'.
'a'+'dive'='adive'
'dit' placed into 'adive' is 'ADDITIVE'.
(Other definitions for additive that I've seen before include "Substance put into food" , "Flavour enhancer" , "Supplement" , "Something put into food" , "Substance stirred in to colour or flavour food" .)