Element, not particularly well done, at huge cost? (4,5)
I believe the answer is:
rare earth
'element' is the definition.
(rare earth is a kind of element)
'not particularly well done at huge cost?' is the wordplay.
'not particularly well done at' becomes 'rare' (I can't explain this - if you can you should believe this answer much more).
'huge cost?' becomes 'earth' (I've seen this before).
'rare'+'earth'='RARE-EARTH'
Can you help me to learn more?
(Other definitions for rare earth that I've seen before include "Particular element" , "One of a group of 15 hard to distinguish elements - at her rear (anag)" , "Oxide" , "Metal" , "One of a particular group of metallic elements" .)