Amount to part of a whole account (6)
I believe the answer is:
behalf
'account' is the definition.
(as in doing something on someone's behalf)
'amount to part of a whole' is the wordplay.
'amount to' becomes 'be' (to 'amount to' is to be or be equivalent to).
'part of a whole' becomes 'half' (one half of a whole).
'be'+'half'='BEHALF'
(Other definitions for behalf that I've seen before include "(Someone's) benefit or sake" , "On ...... of; as reps of" , "On - of means in someone's interest" , "On ________ of, as a representative" , "(A person's) interest" .)