Superlatively flush in Berlin, I fitted in holiday (7)
I believe the answer is:
richest
'superlatively flush' is the definition.
'in berlin i fitted in holiday' is the wordplay.
'in berlin i' becomes 'ich' ('I' in German).
'fitted in' indicates putting letters inside.
'holiday' becomes 'rest' (I've seen this before).
'ich' going within 'rest' is 'RICHEST'.
(Other definitions for richest that I've seen before include "Having the most, money say" , "Least likely to be a beggar" , "most valuable" , "Most affluent and opulent" , "Most productive" .)