Additional payment supported by British and American bank (5)
I believe the answer is:
bonus
'additional payment' is the definition.
(I've seen this in another clue)
'supported by british and american bank' is the wordplay.
'supported by' becomes 'on' (something on something else is supported by it).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'and' means one lot of letters go next to another.
'american' becomes 'us' (abbreviation for United States).
'bank' is an insertion indicator (outside letters create limits or banks).
'b'+'us'='bus'
'on' going into 'bus' is 'BONUS'.
(Other definitions for bonus that I've seen before include "commission" , "Extra payment, may be performance-related" , "Extra payout" , "Extra dividend" , "Extra payment for good work" .)