Invader loses article — that's standard (4)
I believe the answer is:
norm
'that's standard' is the definition.
(norm is a kind of standard)
'invader loses article' is the wordplay.
'invader' becomes 'norman' (as in the Norman Conquest of 1066).
'loses' indicates named letters should be taken away.
'article' becomes 'an' (indefinite article).
'norman' with 'an' taken out is 'NORM'.
(Other definitions for norm that I've seen before include "Mean" , "Required or acceptable standard" , "'Standard, usual (4)'" , "Most frequent state" , "Average result" .)