Unusual blue lava worth a lot (8)
I believe the answer is:
valuable
'worth a lot' is the definition.
(I've seen this before)
'unusual blue lava' is the wordplay.
'unusual' indicates an anagram.
'blue'+'lava'='bluelava'
'bluelava' is an anagram of 'VALUABLE'.
(Other definitions for valuable that I've seen before include "Blue lava may be worth money" , "Of great importance or worth" , "Of considerable worth" , "Expensive, prized" , "Worth a lot, unlike 33 Across" .)