Britain's behind in US money (5)
I believe the answer is:
brass
'money' is the definition.
(I've seen this before)
'britain's behind in us' is the wordplay.
'britain' becomes 'br' (abbreviation).
'behind in us' becomes 'ass' (US term for one's behind).
'br'+'ass'='BRASS'
(Other definitions for brass that I've seen before include "Front" , "Bold self-confidence" , "Yellowish alloy" , "Nerve (but not of steel!)" , "Ready" .)