The colour of gold in the plant (5)
I believe the answer is:
ivory
'the colour' is the definition.
(ivory is a kind of colour)
'gold in the plant' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'in the' means one lot of letters goes inside another.
'plant' becomes 'ivy' (ivy is a plant).
'or' inserted within 'ivy' is 'IVORY'.
'of' is the link.
(Other definitions for ivory that I've seen before include "What tusks are made of" , "Elephant tusk material" , "Material from tusks of certain mammals" , "Yellowish white - material from tusks" , "Key perhaps" .)