Got inside military shelter with English knight left behind (9)
I believe the answer is:
forgotten
'left behind' is the definition.
(something forgotten has been left behind)
'got inside military shelter with english knight' is the wordplay.
'inside' means one lot of letters goes inside another.
'military shelter' becomes 'fort' (type of military building).
'with' is a charade indicator (letters next to each other).
'english' becomes 'E' (abbreviation).
'knight' becomes 'n' (chess).
'fort'+'e'+'n'='forten'
'got' placed into 'forten' is 'FORGOTTEN'.
(Other definitions for forgotten that I've seen before include "Disregarded" , "left behind" , "Neglected" , "Gone from memory" , "Not accessible to memory" .)