Proposition nothing to do with article male pens (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'nothing to do with article male pens' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'to do with' becomes 're' (regarding).
'article' becomes 'the' (the 'definite article' part of speech).
'male' becomes 'm' (common abbreviation).
'pens' means one lot of letters goes inside another (to pen can mean to contain or imprison).
'the'+'m'='them'
'o'+'re'='ore'
'ore' put into 'them' is 'THEOREM'.
(Other definitions for theorem that I've seen before include "Hero met (anag) - principle that's proven" , "Proved mathematical hypothesis" , "Geometric proposition" , "Statement to be proved in maths" , "Proven hypothesis" .)