Deposit balance in Post Office without delay (6)
I believe the answer is:
presto
'without delay' is the definition.
Both the definition and answer are adverbs. Maybe you can see an association between them that I can't see?
'deposit balance in post office' is the wordplay.
'deposit balance' becomes 'rest' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'post office' becomes 'po'.
'rest' inserted within 'po' is 'PRESTO'.
Can you help me to learn more?
(Other definitions for presto that I've seen before include "scored quickly" , "In quick tempo (mus.)" , "rapidly" , "Hey . . . . ., says the magician (6)" , "Hey ..., said the magician" .)