Plain in-house article? (7)
I believe the answer is:
vanilla
'plain' is the definition.
(I've seen this before)
'in-house article?' is the wordplay.
'in' means one lot of letters goes inside another.
'house' becomes 'villa' (villa is a kind of house).
'article?' becomes 'an' (English indefinite article).
'villa' going around 'an' is 'VANILLA'.
(Other definitions for vanilla that I've seen before include "devoid of special features" , "Ordinary" , "ice cream, perhaps" , "Common ice cream flavour" , "Pod that produces flavouring" .)