Foolish detective interrupts boss (6)
I believe the answer is:
stupid
'foolish' is the definition.
('stupid' can be a synonym of 'foolish')
'detective interrupts boss' is the wordplay.
'detective' becomes 'pi' (I've seen this in other clues).
'interrupts' is an insertion indicator.
'boss' becomes 'stud' ('stud' can be a synonym of 'boss').
'pi' inserted inside 'stud' is 'STUPID'.
(Other definitions for stupid that I've seen before include "Not very bright" , "Very silly" , "Brainless" , "Dim" , "Irresponsible" .)