Riddle - "Ultimately, what's kiwi fruit skin?" (4)
I believe the answer is:
sift
'riddle' is the definition.
(riddle can mean to sieve or sift)
'ultimately what's kiwi fruit skin?' is the wordplay.
'ultimately' suggests the final letters.
'skin?' says to hollow out the word (remove centre letters) (I've seen 'skin' mean this (only the skin or outside of the word is used)).
'fruit' with its centre removed is 'ft'.
The final letters of 'whats kiwi' are 'si'.
'si'+'ft'='SIFT'
(Other definitions for sift that I've seen before include "Make fist of filter" , "Get lumps from (flour)" , "Sort carefully" , "Sieve, examine closely" , "screen" .)