Humourless quip of ace detective, in part (2-5)
I believe the answer is:
po-faced
'humourless' is the definition.
(I've seen this before)
'quip of ace detective in part' is the wordplay.
'in part' indicates the answer is hidden within the clue.
'PO-FACED' is hidden in the letters of 'quip of ace detective'.
(Other definitions for po-faced that I've seen before include "With disapproving look" , "Informally solemn in appearance, like nocturnal vessel?" , "Wearing a stern expression" , "Humourlessly disapproving" , "With a disapproving countenance" .)