Two daughters introduced to beer go off (5)
I believe the answer is:
addle
'go off' is the definition.
(eg an egg might addle)
'two daughters introduced to beer' is the wordplay.
'two daughters' means two replacements for 'daughters'.
'daughters' becomes 'd' (genealogical abbreviation for daughters).
'daughters' becomes 'd' (genealogical abbreviation for daughters).
'introduced to' is an insertion indicator.
'beer' becomes 'ale' (type of beer).
'd'+'d'='dd'
'dd' going within 'ale' is 'ADDLE'.
(Other definitions for addle that I've seen before include "Confuse - decay" , "Confuse; become rotten" , "Get rotten" , "Confuse; go off" , "becomes muddled" .)