Foolish private investigator interrupting game of poker (6)
I believe the answer is:
stupid
'foolish' is the definition.
(synonyms)
'private investigator interrupting game of poker' is the wordplay.
'private investigator' becomes 'PI' (abbreviation).
'interrupting' is an insertion indicator.
'game of poker' becomes 'stud' (stud is a version of poker).
'pi' inserted inside 'stud' is 'STUPID'.
(Other definitions for stupid that I've seen before include "Not very bright" , "(Seeming to be) lacking intelligence" , "Dull" , "Barmy" , "Lacking wit or intelligence" .)