Horse clipping top of hurdle (3)
I believe the answer is:
nag
'horse clipping' is the definition.
'nag' can be an answer for 'horse' (term for an old horse). I'm not certain of the 'clipping' bit.
'top of hurdle' is the wordplay.
'top of' means to remove the first letter (I've seen 'top' mean this).
'hurdle' becomes 'snag' (I've seen this before).
'snag' with its first letter taken away is 'NAG'.
Can you help me to learn more?
(Other definitions for nag that I've seen before include "Worry constantly" , "Poor horse" , "Persistently worry; horse" , "to get on one's back" , "Remind constantly" .)