The old rock bands proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'the old rock bands' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rock bands' becomes 'rem' (I can't justify this - if you can you should believe this answer much more).
'the'+'o'+'rem'='THEOREM'
Can you help me to learn more?
(Other definitions for theorem that I've seen before include "Proven hypothesis" , "Algebraic rule" , "The more it changed to a proposition to be proved" , "Deduced formula" , "Proven proposition" .)