Mother foolishly embracing European proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(I know that theorem is a type of proposition)
'mother foolishly embracing european' is the wordplay.
'foolishly' indicates an anagram.
'embracing' is an insertion indicator.
'european' becomes 'E' (abbreviation e.g. EU).
'mother' anagrammed gives 'theorm'.
'theorm' going around 'e' is 'THEOREM'.
(Other definitions for theorem that I've seen before include "A rule in algebra" , "Proven proposition" , "Statement to be proved in maths" , "Proposition not self-evident but proved by a chain of reasoning" , "Deduced formula" .)