Be a toff disguised being eccentric (7)
I believe the answer is:
offbeat
'being eccentric' is the definition.
(offbeat means eccentric or unexpected)
'be a toff disguised' is the wordplay.
'disguised' is an anagram indicator.
'be'+'a'+'toff'='beatoff'
'beatoff' is an anagram of 'OFFBEAT'.
(Other definitions for offbeat that I've seen before include "Quirky" , "Way-out" , "rather unorthodox" , "Wacky; syncopated" , "Unusual, not ordinary - or rhythmic" .)