Quite bad as an illustration (3,3)
I believe the answer is:
ill say
'quite' is the definition.
(I've seen this before)
'bad as an illustration' is the wordplay.
'bad' becomes 'ill' (associated in meaning).
'as an illustration' becomes 'say' (I can't justify this - if you can you should give a lot more credence to this answer).
'ill'+'say'='ILL SAY'
Can you help me to learn more?
(Other definitions for ill say that I've seen before include "Indubitably" , "Absolutely!" , "Definitely" , "Injuries" , "Quite so!" .)