Middle son keeps away from wrongdoer (5)
I believe the answer is:
inner
'middle' is the definition.
(thesaurus)
'son keeps away from wrongdoer' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'keeps away from' is a deletion indicator.
'wrongdoer' becomes 'sinner' (sinner is a kind of wrongdoer).
'sinner' with 's' taken out is 'INNER'.
(Other definitions for inner that I've seen before include "Annulus nearest bull" , "Hidden" , "Esoteric" , "Opposite of outer" , "Part of the bull" .)