Fodder ultimately for the old grey donkey (3,5)
I believe the answer is:
rye grass
'fodder' is the definition.
(I've seen this before)
'ultimately for the old grey donkey' is the wordplay.
'ultimately' suggests the final letters.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'grey' becomes 'gr' (I've seen this in other clues).
'donkey' becomes 'ass' (ass is a term for a donkey).
The final letter of 'for' is 'r'.
'r'+'ye'+'gr'+'ass'='RYE-GRASS'
(Other definitions for rye grass that I've seen before include "lawn" , "Fodder" .)