Smartest One Day International match is entertaining (7)
I believe the answer is:
tidiest
'smartest' is the definition.
('tidy' can be similar in meaning to 'smart')
'one day international match is entertaining' is the wordplay.
'one' becomes 'I' (Roman numeral).
'day' becomes 'd' (abbreviation for day).
'international' becomes 'i' (abbreviation eg in IMF).
'match' becomes 'test' (test match is a kind of match).
'is entertaining' means one lot of letters goes inside another.
'i'+'d'+'i'='idi'
'idi' going into 'test' is 'TIDIEST'.
(Other definitions for tidiest that I've seen before include "One orderly more than others" , "smarter than anything" , "optimally arranged" , "most trim" , "Extremely organised" .)