Recluse's side-splitting joke (5)
I believe the answer is:
loner
'recluse's' is the definition.
(lonely or isolated person)
'side-splitting joke' is the wordplay.
'side' becomes 'l' (abbreviation for left).
'splitting joke' becomes 'oner' (I am not sure about this - if you are sure you should believe this answer much more).
'l'+'oner'='LONER'
(Other definitions for loner that I've seen before include "Solitary type" , "He's not a joiner" , "One liking solitude" , "Individualist" , "One preferring own company" .)