Workplace not on rocks (6)
I believe the answer is:
office
'workplace' is the definition.
(I've seen this in another clue)
'not on rocks' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'rocks' becomes 'ice' (both can informally mean diamonds).
'off'+'ice'='OFFICE'
(Other definitions for office that I've seen before include "Official position" , "Business room or religious rite" , "Position in government?" , "Place for administrative business" , "Position, or building in which colours in layers" .)