Goes on at horses (4)
I believe the answer is:
nags
'horses' is the definition.
(I know this)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for nags that I've seen before include "Constantly finds fault" , "Pesters" , "Constantly reproaches and complains" , "Scolds - horses" , "Worries; horses" .)