Solitary hothead with unusual merit (6)
I believe the answer is:
hermit
'solitary' is the definition.
(a solitary person)
'hothead with unusual merit' is the wordplay.
'hothead' becomes 'h' (head letter of 'hot').
'with' is a charade indicator (letters next to each other).
'unusual' indicates anagramming the letters.
'merit' with letters rearranged gives 'ermit'.
'h'+'ermit'='HERMIT'
(Other definitions for hermit that I've seen before include "Ascetic who leads an isolated life" , "Anchorite, one who lives alone for religious reasons" , "One lives an isolated religious life" , "One lives in solitude for religious reasons" , "Solitary dweller" .)