Maybe rib second-class individual (4)
I believe the answer is:
bone
'maybe rib' is the definition.
(I know that costa is a type of bone)
'second-class individual' is the wordplay.
'second class' becomes 'b'.
'individual' becomes 'one' (an individual is one person).
'b'+'one'='BONE'
(Other definitions for bone that I've seen before include "Ossified material" , "Eg, radius or stirrup" , "Scapula, radius or tibia?" , "How dry" , "Carpal or tarsal, for example" .)