Corrupt document finally isn't (5)
I believe the answer is:
taint
'corrupt' is the definition.
(synonyms)
'document finally isn't' is the wordplay.
'finally' suggests the final letters.
'isn't' becomes 'aint' (non-standard version of 'isn't').
The last letter of 'document' is 't'.
't'+'aint'='TAINT'
(Other definitions for taint that I've seen before include "Adulterate" , "Evidence of pollution" , "black mark" , "It isn't commonly" , "Sully, stain" .)