Roughly, allowance is split (6)
I believe the answer is:
cleave
'is split' is the definition.
(I've seen this before)
'roughly allowance' is the wordplay.
'roughly' becomes 'c' (circa).
'allowance' becomes 'leave' (eg annual leave).
'c'+'leave'='CLEAVE'
(Other definitions for cleave that I've seen before include "Adhere or split apart" , "Split apart - stick together" , "Unite together; split apart" , "Chop or stick to" , "remain attached" .)