Said in passing to be corrupt (10)
I believe the answer is:
demoralise
'corrupt' is the definition.
(I know that corrupt can be written as demoralise)
'said in passing' is the wordplay.
'said' becomes 'oral' (both can mean spoken).
'in' means one lot of letters goes inside another.
'passing' becomes 'demise' (I've seen this before).
'oral' placed within 'demise' is 'DEMORALISE'.
'to be' is the link.
(Other definitions for demoralise that I've seen before include "Cause to lose confidence" , "Make downhearted" , "Throw into confusion" , "Crush" , "Cause to lose confidence/hope" .)