Provoke heartless rider's support (4,2)
I believe the answer is:
stir up
'provoke' is the definition.
(stirring up is a kind of provoking)
'heartless rider's support' is the wordplay.
'heartless' suggests removing the centre.
'rider's support' becomes 'stirrup' (I've seen this before).
'stirrup' with its middle taken out is 'STIR UP'.
(Other definitions for stir up that I've seen before include "Fire in" , "Get excited" , "Instigate" , "Cause or provoke trouble" , "Incite; mix" .)