Turn out an undersized specimen (4)
I believe the answer is:
runt
'an undersized specimen' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'turn out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'turn' anagrammed gives 'RUNT'.
Can you help me to learn more?
(Other definitions for runt that I've seen before include "Smallest piglet of a litter" , "Weak little squealer" , "Smallest animal in a litter" , "Littlest oinker" , "Smallest squealer" .)