Concluding the key to enter it is the smallest (5)
I believe the answer is:
least
'the smallest' is the definition.
(I've seen this before)
'concluding the key to enter' is the wordplay.
'concluding' becomes 'last' (synonyms).
'the key' becomes 'e' (I've seen this in other clues).
'to enter' is an insertion indicator.
'last' going around 'e' is 'LEAST'.
'it is' is the link.
(Other definitions for least that I've seen before include "Minimum" , "Smallest in amount or degree" , "Smallest (in significance)" , "Smallest, slightest" , "Minimal amount" .)