Head onto secondary route — it's showing green (5,4)
I believe the answer is:
broad bean
'green' is the definition.
(green vegetable)
'head onto secondary route' is the wordplay.
'head' becomes 'be an' (bean is a kind of head).
'onto' says to put letters next to each other.
'secondary route' becomes 'b road'.
'bean' put after 'broad' is 'BROAD BEAN'.
'it's showing' is the link.
(Other definitions for broad bean that I've seen before include "plant" , "Green vegetable" .)