Leaving diamonds in business premises (6)
I believe the answer is:
office
'business premises' is the definition.
(I've seen this before)
'leaving diamonds' is the wordplay.
'leaving' becomes 'off' ('I'm off' means 'I'm leaving').
'diamonds' becomes 'ice' (slang term for diamonds).
'off'+'ice'='OFFICE'
'in' is the link.
(Other definitions for office that I've seen before include "Job - place of work" , "Professional or clerical duties are performed here" , "Business room - position of responsibility or duty" , "Administration room" , "Place for administration business" .)