Sort of ache a nuisance but the least costly! (8)
I believe the answer is:
cheapest
'the least costly' is the definition.
I can't tell whether this defines the answer.
'sort of ache a nuisance' is the wordplay.
'sort of' indicates anagramming the letters.
'a nuisance' becomes 'pest' (I've seen this before).
'ache' is an anagram of 'chea'.
'chea'+'pest'='CHEAPEST'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for cheapest that I've seen before include "Set peach in least expensive category" , "Most inferior" , "Lowest in price" , "Most cut-price" , "The space (anag.)" .)