Family doctor with fashionable boss (7)
I believe the answer is:
kingpin
'boss' is the definition.
(I know that kingpin is a type of boss)
'family doctor with fashionable' is the wordplay.
'family' becomes 'kin' (synonyms).
'doctor' becomes 'gp' (general practitioner).
'with' says to put letters next to each other.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'kin'+'gp'+'in'='KINGPIN'
(Other definitions for kingpin that I've seen before include "25' [TOP BANANA]" , "Crucial person in organisation" , "Ring leader" , "See the most important person" , "Supremo" .)