Fool oddly late joining party (4)
I believe the answer is:
dolt
'fool' is the definition.
(thesaurus)
'oddly late joining party' is the wordplay.
'oddly' means one should take alternating letters.
'joining' says to put letters next to each other.
'party' becomes 'do' (do is a kind of party).
The alternating letters of 'late' are 'lt'.
'lt' after 'do' is 'DOLT'.
(Other definitions for dolt that I've seen before include "Dunce" , "Halfwit" , "Stupid person" , "Slow-witted type" , "Idiot" .)