Article indicator of alternative space hypothesis (7)
I believe the answer is:
theorem
'hypothesis' is the definition.
(I've seen this before)
'article indicator of alternative space' is the wordplay.
'article' becomes 'the' (the definite article in English).
'indicator of alternative' becomes 'or'.
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
'the'+'or'+'em'='THEOREM'
(Other definitions for theorem that I've seen before include "A rule in algebra" , "deduction" , "Mathematical rule" , "(Maths) proposition" , "Mathematical axiom" .)