Humorous to turn up, turning detectives (7)
I believe the answer is:
comedic
'humorous' is the definition.
(relating to comedy)
'turn up turning detectives' is the wordplay.
'turn up' becomes 'come' (both can mean to arrive).
'turning' is a reversal indicator.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'cid' back-to-front is 'dic'.
'come'+'dic'='COMEDIC'
'to' is the link.
(Other definitions for comedic that I've seen before include "Funny, laughable" , "Cheerfully entertaining" , "description of light work in theatre" , "Of humour" , "Like a joke" .)