End of loop on rope used to train horses to duck (6)
I believe the answer is:
plunge
'duck' is the definition.
(thesaurus)
'end of loop on rope used to train horses' is the wordplay.
'end of' suggests the final letters.
'on' means one lot of letters go next to another.
'rope used to train horses' becomes 'lunge' (rein used on a horse).
The last letter of 'loop' is 'p'.
'p'+'lunge'='PLUNGE'
'to' acts as a link.
(Other definitions for plunge that I've seen before include "Dive - submerge" , "Sudden drop" , "Move suddenly" , "Dive or fall headlong" , "Pitch" .)