Small measure of weight? More, ultimately (5)
I believe the answer is:
stone
'small' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'measure of weight? more ultimately' is the wordplay.
'measure of' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'weight?' becomes 'ton' (unit of weight).
'ultimately' suggests the final letters.
The final letter of 'more' is 'e'.
's'+'ton'+'e'='STONE'
(Other definitions for stone that I've seen before include "Rock; cherry seed" , "Kernel" , "that one will 26d at 8a [FLING at HIGHLAND GAMES]" , "Rock; fruit seed" , "Seed of e.g. cherry" .)