Those folk must hoard gold: that’s the thinking (6)
I believe the answer is:
theory
'the thinking' is the definition.
(thesaurus)
'those folk must hoard gold' is the wordplay.
I cannot quite understand how this works, but
'gold' could be 'or' ('or' is the heraldic name for gold) and 'or' is found within the answer.
'those' could be 'they' (I've seen this in another clue) and 'they' is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for theory that I've seen before include "General principles, as opposed to practice" , "One with specialised knowledge" , "Hypothesis, thesis" , "Idea for testing" , "General principle which explains some aspect of the world" .)