The smallest allowed as admitted (5)
I believe the answer is:
least
'the smallest' is the definition.
(I've seen this in another clue)
'allowed as admitted' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'admitted' is an insertion indicator (inserted letters are admitted in).
'let' going around 'as' is 'LEAST'.
(Other definitions for least that I've seen before include "Fewest" , "Smallest part" , "Most slight" , "Steal (anag)" , "Opposite of most" .)