Realise English includes Shakespeare material (9)
I believe the answer is:
gabardine
'material' is the definition.
(gabardine is a kind of material)
'realise english includes shakespeare' is the wordplay.
'realise' becomes 'gain' ('gain' can be a synonym of 'realize').
'english' becomes 'e' (abbreviation).
'includes' indicates putting letters inside.
'shakespeare' becomes 'bard' (I've seen this before).
'gain'+'e'='gaine'
'gaine' going around 'bard' is 'GABARDINE'.
(Other definitions for gabardine that I've seen before include "durable fabric" , "raincoat" , "Bargained for strong fabric" , "cloth" , "material" .)