Controversial Italian banker joins mile high clubs (7)
I believe the answer is:
polemic
'controversial' is the definition.
(similar in meaning)
'italian banker joins mile high clubs' is the wordplay.
'italian banker' becomes 'po' (Italian river, something that 'banks').
'joins' is an insertion indicator (inserted letters join the word).
'high' indicates anagramming the letters (like drunk).
'clubs' becomes 'c' (abbreviation in card games).
'mile' anagrammed gives 'lemi'.
'po' placed inside 'lemi' is 'polemi'.
'polemi'+'c'='POLEMIC'
(Other definitions for polemic that I've seen before include "Involving dispute or controversy" , "Strong verbal or written attack" , "Controversy, argument" , "Hostile argument" , "debate" .)