Form a clump (4)
I believe the answer is:
clot
'form a clump' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clot that I've seen before include "Twit; blood problem" , "Mass of material" , "Clumsy person" , "Coagulate - fool" , "Idiot - liquid turned to a solid state" .)