Hermit contributing to phrenology overhaul (5)
I believe the answer is:
loner
'hermit' is the definition.
(I've seen this in other clues)
'contributing to phrenology overhaul' is the wordplay.
'contributing to' indicates the answer is hidden within the clue (the answer contributes to part of the clue ).
'overhaul' shows that the letters should be reversed in order (I've seen 'overhauled' mean this).
'renol' is hidden in the letters of 'phrenology'.
'renol' in reverse letter order is 'LONER'.
(Other definitions for loner that I've seen before include "Solitary person" , "No mixer" , "One shunning company" , "Hermit" , "One lacking friends" .)