Head of colour mixed paint in plant (6)
I believe the answer is:
catnip
'plant' is the definition.
(catnip is a kind of plant)
'head of colour mixed paint' is the wordplay.
'head of' indicates taking the first letters.
'mixed' indicates an anagram.
The initial letter of 'colour' is 'c'.
'paint' with letters rearranged gives 'atnip'.
'c'+'atnip'='CATNIP'
'in' is the link.
(Other definitions for catnip that I've seen before include "feline favourite?" , "flower" , "Felines fancy it" , "'Hairy herb, strongly attractive to felines (6)'" , "treat for moggie" .)