Solver finally cracks nastier sort of riddle (8)
I believe the answer is:
strainer
'riddle' is the definition.
(riddle can mean a sieve or strainer)
'solver finally cracks nastier sort' is the wordplay.
'finally' says to take the final letters.
'cracks' means one lot of letters goes inside another (crack their way in).
'sort' indicates an anagram.
The final letter of 'solver' is 'r'.
'nastier' anagrammed gives 'stainer'.
'r' inserted into 'stainer' is 'STRAINER'.
'of' acts as a link.
(Other definitions for strainer that I've seen before include "Tea leaf remover" , "one full of holes" , "Kitchen utensil" , "Retrains (anag.)" , "One trying very hard" .)