Dim boss recruits gumshoe (6)
I believe the answer is:
stupid
'dim' is the definition.
(similar in meaning)
'boss recruits gumshoe' is the wordplay.
'boss' becomes 'stud' (both can mean a metal decoration).
'recruits' means one lot of letters goes inside another.
'gumshoe' becomes 'PI' (private investigator - gumshoe means detective).
'stud' enclosing 'pi' is 'STUPID'.
(Other definitions for stupid that I've seen before include "Senseless" , "Ass" , "Not sensible or intelligent" , "Brainless" , "Unintelligent, really thick" .)