Get on with horse, stifling feeling of irritation (5,4)
I believe the answer is:
pitch into
'get on' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.
'horse stifling feeling of irritation' is the wordplay.
'horse' becomes 'pinto' (pinto is a kind of horse).
'stifling' indicates putting letters inside (some letters get stifled or contained).
'feeling of irritation' becomes 'itch'.
'pinto' placed around 'itch' is 'PITCH INTO'.
'with' is the link.
Can you help me to learn more?
(Other definitions for pitch into that I've seen before include "Attack" , "Vigorously assail (someone)" .)