Expose corrupt man in American bank, finally (6)
I believe the answer is:
unmask
'expose' is the definition.
(unmasking is a kind of exposing)
'corrupt man in american bank finally' is the wordplay.
'corrupt' indicates an anagram.
'in' is an insertion indicator.
'american' becomes 'us' (**).
'finally' indicates one should take the final letters.
The final letter of 'bank' is 'k'.
'man' anagrammed gives 'nma'.
'nma' inserted into 'us' is 'unmas'.
'unmas'+'k'='UNMASK'
(Other definitions for unmask that I've seen before include "Expose, remove face covering" , "Discover the identity of" , "Disclose (the hidden nature of)" , "Expose the true character of (a person)" , "Identify (someone hidden)" .)