Is old mattress I found in hollow breaking the rules? (11)
I believe the answer is:
disobedient
'breaking the rules?' is the definition.
'is old mattress i found in hollow' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mattress' becomes 'bed'.
'found in' is an insertion indicator.
'hollow' becomes 'dent' (both can mean an impression or indentation).
'is'+'o'+'bed'+'i'='isobedi'
'isobedi' going inside 'dent' is 'DISOBEDIENT'.
(Other definitions for disobedient that I've seen before include "Ignoring instructions" , "Rebellious" , "Naughty" , "Going against instructions" .)