Band-Aid left out by veterinarian, at last, for part for horse's foot (7)
I believe the answer is:
pastern
'for part for horse's foot' is the definition.
(sloping part of a horse's foot)
'band-aid left out by veterinarian at' is the wordplay.
'band-aid' becomes 'plaster' (Band-Aid is a brand of plaster).
'left' becomes 'L' (common abbreviation).
'out' is a deletion indicator.
'by veterinarian at' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'plaster' with 'l' removed is 'paster'.
'paster'+'n'='PASTERN'
'last' acts as a link.
This may not be correct. It may belong to another bit of the clue.
(Other definitions for pastern that I've seen before include "Part of horse's foot" , "part of Arab's leg" , "'The knee of an horse' (Johnson)" , "Fetter-bone of horse" , "Wrongly-defined part of horse" .)