Proposition made by Greek boy concerning end of term? (7)
'proposition' is the definition.
(theorem is a kind of proposition)
'greek boy concerning end of term?' is the wordplay.
'greek boy' becomes 'theo' (common Greek name).
'concerning' becomes 're'.
'end of' says to take the final letters.
The final letter of 'term' is 'm'.
'made by' acts as a link.
(Other definitions for theorem that I've seen before include "Demonstrable proposition", "Proven proposition", "Mathematical proposition proved by reasoning", "Mathematical formula", "Logical proposition".)