There's love in monsieur's proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'there's love in monsieur's' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'in' indicates putting letters inside.
'monsieur' becomes 'm'.
'there' enclosing 'o' is 'theore'.
'theore'+'m'='THEOREM'
(Other definitions for theorem that I've seen before include "speculation" , "Mathematical formula" , "Proof" , "Statement to be proved in maths" , "Proven proposition" .)