Source of flavour in beer — the culminating touch (6)
I believe the answer is:
finale
'the culminating touch' is the definition.
(the final part of something or a final flourish)
'source of flavour in beer' is the wordplay.
'source of' suggests taking the first letters.
'beer' becomes 'ale' (type of beer).
The first letter of 'flavour' is 'f'.
'f'+'in'+'ale'='FINALE'
(Other definitions for finale that I've seen before include "Conclusion of a performance" , "Last part of something" , "Final section of a musical composition" , "Denouement" , "Last section of a musical composition" .)