Policeman gets errant criminal as colleague (9)
I believe the answer is:
copartner
'colleague' is the definition.
(thesaurus)
'policeman gets errant criminal' is the wordplay.
'policeman' becomes 'cop' (I've seen this before).
'gets' means one lot of letters go next to another.
'criminal' is an anagram indicator.
'errant' is an anagram of 'artner'.
'cop'+'artner'='COPARTNER'
'as' acts as a link.
(Other definitions for copartner that I've seen before include "Associate with equal share in a business" , "fellow to work with as an equal?" .)