A bitter's served up to rock bands? (6)
I believe the answer is:
strata
'rock bands?' is the definition.
(I've seen this in another clue)
'a bitter's served up' is the wordplay.
'bitter's' becomes 'tart's' (tart can mean bitter-tasting).
'served up' shows that the letters should be reversed in order (in a down clue, going upwards).
'a'+'tarts'='atarts'
'atarts' reversed gives 'STRATA'.
'to' acts as a link.
(Other definitions for strata that I've seen before include "Parallel layers" , "bands" , "Layers of sedimentary rock" , "Levels, grades or social classes" , "Layers of rock on earth's surface" .)