Strain is breaking musical group that's laid back (7)
I believe the answer is:
tension
'strain' is the definition.
(I know that tension is a type of strain)
'is breaking musical group that's laid back' is the wordplay.
'breaking' indicates putting letters inside.
'musical group' becomes 'nonet' (group of nine musicians).
'that's laid back' shows that the letters should be reversed in order.
'is' inserted inside 'nonet' is 'noisnet'.
'noisnet' in reverse letter order is 'TENSION'.
(Other definitions for tension that I've seen before include "The effect of force exerted in or on something" , "Assay" , "State of strain or suspense" , "Stretched state" , "One isn't (anag.)" .)