Intelligence reduced by not working (3-4)
I believe the answer is:
low-down
'intelligence' is the definition.
(thesaurus)
'reduced by not working' is the wordplay.
'reduced' becomes 'low' ('low' can be similar in meaning to 'reduced'**).
'by' is a charade indicator (letters next to each other).
'not working' becomes 'down'.
'low'+'down'='LOW-DOWN'
(Other definitions for low-down that I've seen before include "Inside story" , "Inside information - mean (7;3-4)" , "Data - base" , "Info (slang) - dishonourable" , "Mean, unfair" .)