Turn out to be the smallest (4)
I believe the answer is:
runt
'be the smallest' is the definition.
(I've seen this in another clue)
'turn out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'turn' is an anagram of 'RUNT'.
'to' is the link.
(Other definitions for runt that I've seen before include "Weak individual" , "Smallest pig in litter" , "Littlest oinker" , "Smallest animal in a litter" , "Litter-weakling" .)