Expression could be sans forced admission (5)
I believe the answer is:
idiom
'expression' is the definition.
(idiom is a kind of expression)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for idiom that I've seen before include "Saying specific to a certain language" , "Dialect, vernacular" , "Characteristic language of people or group" , "Characteristic mode of expression" , "Language used by people or group, vernacular" .)