Small, ultimately unwanted plant (4)
I believe the answer is:
weed
'plant' is the definition.
(A weed is a plant)
'small ultimately unwanted' is the wordplay.
'small' becomes 'wee' (Scottish term for small).
'ultimately' indicates one should take the final letters.
The last letter of 'unwanted' is 'd'.
'wee'+'d'='WEED'
(Other definitions for weed that I've seen before include "Work in garden" , "Plant growing in the wrong place!" , "Intrusive plant; weak person" , "Plant growing where it is not wanted" , "Marijuana (slang)" .)