Explain allowance is included in beer (11)
I believe the answer is:
rationalise
'explain' is the definition.
('explain'->'rationalize' is in my internal thesaurus and 'ize'->'ise')
'allowance is included in beer' is the wordplay.
'allowance' becomes 'ration' (I've seen this before**).
'included in' means one lot of letters goes inside another.
'beer' becomes 'ale' (I've seen this before).
'is' placed inside 'ale' is 'alise'.
'ration'+'alise'='RATIONALISE'
(Other definitions for rationalise that I've seen before include "Justify with logic" , "Reorganise logically" .)