Article on maths initially covering old formula (7)
I believe the answer is:
theorem
'formula' is the definition.
(I've seen this before)
'article on maths initially covering old' is the wordplay.
'article' becomes 'the' (the definite article in English).
'on' becomes 're' ('on' can be mean 'with reference to').
'initially' indicates taking the first letters.
'covering' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The initial letter of 'maths' is 'm'.
'the'+'re'+'m'='therem'
'therem' enclosing 'o' is 'THEOREM'.
(Other definitions for theorem that I've seen before include "Logical proposition" , "Mathematical proposition proved by reasoning" , "Deduced formula" , "Proposition not self-evident but proved by a chain of reasoning" , "The more it changed to a proposition to be proved" .)