Corrupt eastern firm (7)
I believe the answer is:
earnest
'firm' is the definition.
(I've seen this before)
'corrupt eastern' is the wordplay.
'corrupt' indicates anagramming the letters (a corrupted spelling of the word).
'eastern' with letters rearranged gives 'EARNEST'.
(Other definitions for earnest that I've seen before include "See 1" , "Serious - advance" , "Firm" , "Serious; down-payment" , "promise" .)