Fight on horseback with no leader to unseat (4)
I believe the answer is:
oust
'to unseat' is the definition.
(I've seen this in another clue)
'fight on horseback with no leader' is the wordplay.
'fight on horseback' becomes 'joust' (synonyms).
'with no leader' means to remove the first letter.
'joust' with its initial letter taken off is 'OUST'.
(Other definitions for oust that I've seen before include "Expel - eject" , "Send packing" , "Drive out, eject from office" , "Remove or reject from position" , "Force from office" .)