A poet was ahead, backing a horse (2,2,4)
I believe the answer is:
de la mare
'a poet' is the definition.
(I've seen this before)
'ahead backing a horse' is the wordplay.
'ahead' becomes 'led' (to lead is to go ahead).
'backing' is a reversal indicator.
'horse' becomes 'mare' (mare is a kind of horse).
'led' in reverse letter order is 'del'.
'del'+'a'+'mare'='DE LA MARE'
'was' acts as a link.
(Other definitions for de la mare that I've seen before include "Walter -, Eng. poet" , "Writer of verse" , "Walter -, English poet" , "Walter -- -- --, author" .)