Talk idly about costume -- worry's over knitted undies (10)
I believe the answer is:
balbriggan
'knitted undies' is the definition.
(I've seen this before)
'talk idly about costume worry's' is the wordplay.
I cannot really see how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is found within the answer.
'costume' could be 'rig' (I've seen this in other clues) and 'rig' is present in the answer.
This explanation may well be incorrect...
'over' is the link.
Can you help me to learn more?
(Another definition for balbriggan that I've seen is " knitted undies".)