I could go on horse to make an impression (4)
I believe the answer is:
etch
'make an impression' is the definition.
(to etch is to make an impression in something)
'i could go on horse' is the wordplay.
I cannot really understand how this works, but
'horse' could be 'h' (abbreviation as in horse power) and 'h' is present in the answer.
The remaining letters 'etc' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for etch that I've seen before include "Incribe" , "Engrave (glass/metal)" , "Chase" , "Inscribe" , "Cut design onto metal using acid" .)