Steep quarry encloses one cave's opening (6)
I believe the answer is:
pricey
'steep' is the definition.
(thesaurus)
'quarry encloses one cave's opening' is the wordplay.
'quarry' becomes 'prey' (synonyms).
'encloses' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'opening' says to take the initial letters.
The initial letter of 'caves' is 'c'.
'i'+'c'='ic'
'prey' going around 'ic' is 'PRICEY'.
(Other definitions for pricey that I've seen before include "Expensive" , "Costly" .)