Matter for thought (5)
I believe the answer is:
brain
I believe this clue is a double definition.
'matter' is the first definition.
(brain is a kind of matter)
'thought' is the second definition.
(thesaurus)
'for' is the link.
(Other definitions for brain that I've seen before include "Clever type" , "to get hit over the head" , "Intelligent person" , "Clever person" , "Thinking thing" .)