Great disappointment about English not on top (5)
I believe the answer is:
below
'not on top' is the definition.
(down below rather than up high)
'great disappointment about english' is the wordplay.
'great disappointment' becomes 'blow' (blow can mean a setback or difficulty).
'about' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'blow' going around 'e' is 'BELOW'.
(Other definitions for below that I've seen before include "Beneath" , "on earth" , "Feel miserable" , "Later in text" , "as mentioned later" .)