Ties rocking horse's tail with pin (13)
I believe the answer is:
relationships
'ties' is the definition.
(tie can mean a relationship or connection)
'rocking horse's tail with pin' is the wordplay.
'rocking' indicates anagramming the letters.
'with' is a charade indicator (letters next to each other).
'horses'+'tail'='horsestail'
'horsestail' after 'pin' is 'pinhorsestail'.
'pinhorsestail' with letters rearranged gives 'RELATIONSHIPS'.
(Other definitions for relationships that I've seen before include "Sexual liaisons" , "Connections" .)