Backing one-pound operas is foolish (10)
I believe the answer is:
blithering
'foolish' is the definition.
(I've seen this before)
'backing one-pound operas' is the wordplay.
'backing' shows that the letters should be reversed in order.
'one' becomes 'i' (roman numeral).
'pound' becomes 'lb' (synonyms).
'operas' becomes 'thering' (The Ring, cycle of operas by Richard Wagner).
'i'+'lb'='ilb'
'ilb' written backwards gives 'bli'.
'bli'+'thering'='BLITHERING'
'is' is the link.
(Other definitions for blithering that I've seen before include "Stupid" , "Foolish" , "talking foolishly" , "Contemptible" .)