Allowed because included the smallest (5)
I believe the answer is:
least
'the smallest' is the definition.
(I've seen this before)
'allowed because included' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'because' becomes 'as' (I've seen this in other clues).
'included' means one lot of letters goes inside another.
'let' going around 'as' is 'LEAST'.
(Other definitions for least that I've seen before include "Smallest, being stale" , "No more petty" , "Bottom" , "Lowest or smallest" , "Most insignificant" .)