Not on, having diamonds in the workplace? (6)
I believe the answer is:
office
'the workplace?' is the definition.
(thesaurus)
'not on having diamonds' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'having' means one lot of letters go next to another.
'diamonds' becomes 'ice' (I've seen this before).
'off'+'ice'='OFFICE'
'in' acts as a link.
(Other definitions for office that I've seen before include "Business premises" , "Place for administrative business" , "base?" , "Administration room" , "Official position" .)