Allowed to go round because smallest (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'allowed to go round because' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'go round' is an insertion indicator.
'because' becomes 'as' (I've seen this in other clues).
'let' enclosing 'as' is 'LEAST'.
(Other definitions for least that I've seen before include "Most unimportant" , "Smallest, being stale" , "Smallest, slightest" , "Bottom" , "superlatively small" .)