Set one’s heart, say, on a material (7)
I believe the answer is:
organon
'set one's' is the definition.
'organon' can be an answer for 'set' (I have seen 'Set of principles' mean 'organon' so perhaps 'set' could also mean 'organon'). I'm not sure about the 'one's' bit.
'heart say on a material' is the wordplay.
'heart say' becomes 'organ' (heart is a kind of organ).
'on' is a charade indicator (letters next to each other).
'a material' becomes 'on' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'organ'+'on'='ORGANON'
Can you help me to learn more?
(Other definitions for organon that I've seen before include "system for acquiring knowledge" , "Set of principles" , "system of logic" .)