Do a shifty job in poor illumination (9)
I believe the answer is:
moonlight
'do a shifty job' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'poor illumination' is the wordplay.
'poor' becomes 'moon' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'illumination' becomes 'light' (synonyms).
'moon'+'light'='MOONLIGHT'
'in' is the link.
Can you help me to learn more?
(Other definitions for moonlight that I've seen before include "do an extra job" , "Illumination from the earth's satellite" , "Natural (and romantic?) illumination" , "Perform work at night" , "work illegally" .)