Athlete on mountain eating a vegetable (6,4)
I believe the answer is:
runner bean
'vegetable' is the definition.
(runner bean is a kind of vegetable)
'athlete on mountain eating a' is the wordplay.
'athlete' becomes 'runner' (type of athlete).
'on' says to put letters next to each other.
'mountain' becomes 'ben' (as in Ben Nevis).
'eating' is an insertion indicator.
'ben' going around 'a' is 'bean'.
'runner'+'bean'='RUNNER-BEAN'
(Other definitions for runner bean that I've seen before include "Green vegetable" , "climber" , "Food item" , "One trained" .)