Solitary hothead with unusual merit (6)
I believe the answer is:
hermit
'solitary' is the definition.
(thesaurus)
'hothead with unusual merit' is the wordplay.
'hothead' becomes 'h' (head letter of 'hot').
'with' says to put letters next to each other.
'unusual' is an anagram indicator.
'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" .)