Admission could be sans any expression (5)
I believe the answer is:
idiom
'expression' is the definition.
(idiom is a kind of expression)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for idiom that I've seen before include "Special turn of phrase" , "Peculiarity of phrase in a language" , "Phrase such as over the moon" , "Distinctive speech" , "'Break a leg' or 'miss the boat'" .)