Mischievous fellow arranged lunch with a peer (10)
I believe the answer is:
leprechaun
'mischievous fellow' is the definition.
'arranged lunch with a peer' is the wordplay.
'arranged' indicates an anagram.
'with' says to put letters next to each other.
'lunch'+'a'+'peer'='lunchapeer'
'lunchapeer' anagrammed gives 'LEPRECHAUN'.
(Other definitions for leprechaun that I've seen before include "Mischievous Irish elf" , "Mischievous imp" , "Mischievous Irish sprite" , "Elf in Irish folklore" , "He helps Irish housewife" .)