Clump together (4)
I believe the answer is:
clot
'clump together' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for clot that I've seen before include "A lump in a blood vessel" , "(Of blood) coagulate" , "3 [DUMMY]" , "Coagulate -- 1 across person" , "Stupid person - lump of blood" .)