Stick together with fool (4)
I believe the answer is:
clot
This clue is a double definition.
'stick together' is the first definition.
(I've seen this in another clue)
'fool' is the second definition.
(I've seen this before)
'with' is the link.
(Other definitions for clot that I've seen before include "Mass of material" , "Semi-solid mass -- dunce" , "Twit; blood problem" , "Idiot - liquid turned to a solid state" , "Thick lump - thick person" .)