Material English teacher understood (8,2)
I believe the answer is:
cottoned on
'understood' is the definition.
(to cotton on to something is to understand it)
'material english teacher' is the wordplay.
'material' becomes 'cotton' (cotton is a kind of material).
'english' becomes 'e' (abbreviation).
'teacher' becomes 'don' (don is a kind of teacher).
'cotton'+'e'+'don'='COTTONED ON'
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