Fool lowers credit (8)
I believe the answer is:
dipstick
'fool' is the definition.
(I know that fool can be written as dipstick)
'lowers credit' is the wordplay.
'lowers' becomes 'dips' (dipping is a kind of lowering).
'credit' becomes 'tick' (I've seen this before).
'dips'+'tick'='DIPSTICK'
(Other definitions for dipstick that I've seen before include "Oil gauge" , "Fluid measuring rod" , "Device for checking oil level" , "one concealed under bonnet?" , "Wally" .)