Informally, it is not corrupt? (5)
I believe the answer is:
taint
This clue is a double definition.
'informally it is not' is the first definition.
('t ain't - informal way of saying 'it isn't')
'corrupt?' is the second definition.
(I know that corrupt can be written as taint)
(Other definitions for taint that I've seen before include "Corrupt" , "(Trace of) corruption" , "Stigmatise" , "Sully, stain" , "Blemish" .)