Seed that is on headgear (6)
I believe the answer is:
beanie
'headgear' is the definition.
(beanie is a kind of headgear)
'seed that is on' is the wordplay.
'seed' becomes 'bean' (bean is a kind of seed).
'that is' becomes 'ie' (synonyms).
'on' says to put letters next to each other.
'bean'+'ie'='BEANIE'
(Other definitions for beanie that I've seen before include "Small close-fitting hat" , "cap" , "Topping" , "one on the head" .)