Mistakenly eats perch (4)
I believe the answer is:
seat
'perch' is the definition.
(thesaurus)
'mistakenly eats' is the wordplay.
'mistakenly' indicates an anagram.
'eats' anagrammed gives 'SEAT'.
(Other definitions for seat that I've seen before include "Somewhere to sit" , "Parliamentary constituency" , "MP's place in parliament" , "Place to sit" , "Chair, bench, saddle, etc" .)