A couple on horse, say, for a second (6,3)
I believe the answer is:
number two
'for a second' is the definition.
(I've seen this before)
'a couple on horse' is the wordplay.
'a couple' becomes 'two' (two people).
'on' says to put letters next to each other.
'horse' becomes 'number' (I am not sure about this - if you are sure you should believe this answer much more).
'two' after 'number' is 'NUMBER TWO'.
'say' is the link.
Can you help me to learn more?
(Other definitions for number two that I've seen before include "Second in command" , "deputy" , "Second-in-command" , "vice" , "Assistant" .)