That of the thistle, for instance, is gold with a red back (5)
I believe the answer is:
order
'that of the thistle for instance' is the definition.
I can't tell whether this definition defines the answer.
'gold with a red back' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'with' is a charade indicator (letters next to each other).
'back' says the letters should be written backwards.
'red' in reverse letter order is 'der'.
'or'+'der'='ORDER'
'is' is the link.
Can you help me to learn more?
(Other definitions for order that I've seen before include "Jesuits, say" , "Command or request for goods" , "Kind - sort" , "Edict, command" , "Tidiness - that's a command" .)