A feature of summer headgear worn by English (4)
I believe the answer is:
heat
'a feature' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'summer headgear worn by english' is the wordplay.
'summer headgear' becomes 'hat' (hats are a type of headgear. I am not sure about the 'summer' bit.).
'worn by' indicates putting letters inside (outside letters are worn by those on the inside).
'english' becomes 'E' (abbreviation).
'hat' placed around 'e' is 'HEAT'.
'of' is the link.
Can you help me to learn more?
(Other definitions for heat that I've seen before include "High temperature" , "Preliminary race to qualify finalists" , "Raise temperature" , "Energy that causes a rise in temperature" , "Provide warmth" .)