Poles nicking silver - and horses! (4)
I believe the answer is:
nags
'horses' is the definition.
(I know this)
'poles nicking silver' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'nicking' means one lot of letters goes inside another (I've seen this in other clues).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'ns' enclosing 'ag' is 'NAGS'.
'and' is the link.
(Other definitions for nags that I've seen before include "Scolds continually" , "Constantly finds fault" , "Pesters" , "Scolds constantly" , "Annoys" .)