Proposition made by Greek boy band (7)
I believe the answer is:
theorem
'proposition' is the definition.
(I know that theorem is a type of proposition)
'by greek boy band' is the wordplay.
'by' says to put letters next to each other.
'greek boy' becomes 'theo' (common Greek name).
'band' becomes 'rem' (REM are a US rock band).
'theo'+'rem' is 'THEOREM'.
'made' is the link.
(Other definitions for theorem that I've seen before include "Hero met (anag) - principle that's proven" , "this has been established" , "speculation" , "Mathematical rule" , "Geometrical proposition to be proved" .)