About to climb on horse (7)
I believe the answer is:
remount
'horse' is the definition.
(I know that remount is a type of mount)
'about to climb on' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to' means one lot of letters go next to another.
'climb on' becomes 'mount' (synonyms).
're'+'mount'='REMOUNT'
(Other definitions for remount that I've seen before include "stage again" , "Get back on a horse" .)