Appearance in base announced (4)
I believe the answer is:
mien
'appearance' is the definition.
(thesaurus)
'base announced' is the wordplay.
'base' becomes 'mean' ('mean' can be a synonym of 'base').
'announced' indicates a 'sounds like' (homophone) clue.
'mean' sounds like 'MIEN'.
'in' acts as a link.
(Other definitions for mien that I've seen before include "Appearance, bearing" , "A face like mine" , "A face - maybe mine" , "(One's) manner, bearing" , "person's look" .)