Show off extremely faithful relative (6)
I believe the answer is:
flaunt
'show off' is the definition.
(synonyms)
'extremely faithful relative' is the wordplay.
'extremely' means to remove the middle letters (extreme, outside letters only).
'relative' becomes 'aunt' (aunt is a kind of relative).
'faithful' with its centre removed is 'fl'.
'fl'+'aunt'='FLAUNT'
(Other definitions for flaunt that I've seen before include "wave" , "What's displayed in Globe" , "Display ostentatioulsy, brandish" , "Display ostentatiously or boastfully" , "Show proudly, provocatively" .)