About to beat famous racehorse (3,3)
I believe the answer is:
red rum
'famous racehorse' is the definition.
(three time winner of the Grand National)
'about to beat' is the wordplay.
'about' becomes 're' (regarding).
'to beat' becomes 'drum' (synonyms).
're'+'drum'='RED RUM'
(Other definitions for red rum that I've seen before include "old racehorse" , "Aintree winner" , "Famous racehorse" , "Famous horse" , "Three-time Grand National winner, d. 1995 aged 30" .)