A letter, it's said, a horse may understand (3)
I believe the answer is:
gee
'a letter' is the definition.
(G)
'it's said a horse may understand' is the wordplay.
I cannot really understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
The remaining letters 'ge' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for gee that I've seen before include "Mild exclamation" , "Gracious" , "provided by substitute for ginger?" , "7th letter" , "- - whiz! (US)" .)