Potential grub for horse — eats food round end of day (8)
I believe the answer is:
haystack
'potential grub for horse' is the definition.
'eats food round end of day' is the wordplay.
'eats' becomes 'has' (I've seen this before).
'food' becomes 'tack' (I've seen this before**).
'round' means one lot of letters goes inside another.
'end of' indicates one should take the final letters.
The last letter of 'day' is 'y'.
'has'+'tack'='hastack'
'hastack' enclosing 'y' is 'HAYSTACK'.
(Other definitions for haystack that I've seen before include "Pile of cut grass" , "Large mound of dried grass" , "Rick of dried grass" , "Pile of dried grass" , "Large pile of fodder" .)