Department investigating contracted detective's retirement (3)

'department investigating' is the definition.
(Criminal Investigation Department)

'contracted detective's retirement' is the wordplay.
'contracted' means to remove the last letter (to contract is to shorten).
'detective' becomes 'dick' (informal term for a detective).
'retirement' is a reversal indicator (retire can mean to return or go back).
'dick' with its last letter taken off is 'dic'.
'dic' back-to-front is 'CID'.