Mike opens fizzy stout, the greatest (6)
I believe the answer is:
utmost
'the greatest' is the definition.
(I've seen this before)
'mike opens fizzy stout' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'opens' indicates putting letters inside (one set of letters makes an 'opening' in another).
'fizzy' indicates anagramming the letters (I've seen this in other clues).
'stout' is an anagram of 'utost'.
'm' put inside 'utost' is 'UTMOST'.
(Other definitions for utmost that I've seen before include "Furthest or extreme" , "Maximum effort" , "Paramount" , "(To the) greatest possible extent" , "Furthest or maximum possible" .)