Warning from heads of Ofsted mentioned extra numeracy (4)
I believe the answer is:
omen
'warning' is the definition.
(I've seen this before)
'heads of ofsted mentioned extra numeracy' is the wordplay.
'heads of' indicates taking the first letters.
The first letters of 'ofsted mentioned extra numeracy' is 'OMEN'.
'from' acts as a link.
(Other definitions for omen that I've seen before include "Sign of things to come" , "Warning - sign" , "Augur" , "Sign of likely future event" , "Augury, portent" .)