Obscure form of lament (6)
I believe the answer is:
mantle
'obscure' is the definition.
(thesaurus)
'form of lament' is the wordplay.
'form of' indicates an anagram (another form of the same letters).
'lament' anagrammed gives 'MANTLE'.
(Other definitions for mantle that I've seen before include "Gas lamp part; cloak" , "Robe or gas-lamp fitting" , "Cloak and shelf over the fireplace" , "Cloak or loose outer garment" , "Loose outer garment or covering" .)