Immature island cattle ultimately eating tall grass (8)
I believe the answer is:
inchoate
'immature' is the definition.
(I've seen this before)
'island cattle ultimately eating tall grass' is the wordplay.
'island' becomes 'inch' (I've seen this before**).
'ultimately' says to take the final letters.
'eating' means one lot of letters goes inside another.
'tall grass' becomes 'oat'.
The last letter of 'cattle' is 'e'.
'inch'+'e'='inche'
'inche' going around 'oat' is 'INCHOATE'.
(Other definitions for inchoate that I've seen before include "Imperfectly formed" , "Just begun, not yet fully developed" , "Not established" , "just beginning" , "Action he took having just begun" .)