Join idiot in conversation? (4)
I believe the answer is:
knit
'join' is the definition.
(knitting is a kind of joining)
'idiot in conversation?' is the wordplay.
'idiot' becomes 'nit' (nit can mean a stupid person).
'in conversation?' indicates a 'sounds like' (homophone) clue (I've seen 'in conversation' mean this).
'nit' sounds like 'KNIT'.
(Other definitions for knit that I've seen before include "Form by looping and entwining wool etc" , "Link closely together" , "Make woollen objects" , "Interweave" , "Join together (with needles)" .)