Pressure to witter on about English introduction (8)
I believe the answer is:
preamble
'introduction' is the definition.
(preamble is a kind of introduction)
'pressure to witter on about english' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'to witter on' becomes 'ramble' (both can mean to talk aimlessly).
'about' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'ramble' placed around 'e' is 'reamble'.
'p'+'reamble'='PREAMBLE'
(Other definitions for preamble that I've seen before include "Overture" , "Preliminary or introductory statement" , "Opening words" , "Preface, opening statement" , "Explanatory foreword" .)