Potter's friend mostly solitary and single (8)
I believe the answer is:
hermione
'potter's friend' is the definition.
(Harry Potter)
'mostly solitary and single' is the wordplay.
'mostly' means to remove the last letter.
'solitary' becomes 'hermit' (a solitary person).
'and' means one lot of letters go next to another.
'single' becomes 'one' (synonyms).
'hermit' with its final letter taken off is 'hermi'.
'hermi'+'one'='HERMIONE'
(Other definitions for hermione that I've seen before include "Girl's name" , "Statuesque queen" , "Leontes' queen (Winter's Tale)" .)