Sensible allowance, nearly everything! (8)
I believe the answer is:
rational
'sensible' is the definition.
('rational' is associated in meaning with 'sensible')
'allowance nearly everything' is the wordplay.
'allowance' becomes 'ration' (I've seen this before**).
'nearly' means to remove the last letter.
'everything' becomes 'all' (I've seen this before).
'all' with its final letter removed is 'al'.
'ration'+'al'='RATIONAL'
(Other definitions for rational that I've seen before include "Consistent with logic" , "Not absurd" , "Reasonable - sane" , "Consistent with reason" , "Sensible, based on reason" .)