Surprisingly stout member included is extreme (6)
I believe the answer is:
utmost
'extreme' is the definition.
('utmost' can be a synonym of 'extreme')
'surprisingly stout member included' is the wordplay.
'surprisingly' indicates anagramming the letters.
'member' becomes 'm'.
'included' is an insertion indicator.
'stout' is an anagram of 'utost'.
'utost' placed around 'm' is 'UTMOST'.
'is' is the link.
(Other definitions for utmost that I've seen before include "(To the) maximum extent" , "'Most extreme, greatest (6)'" , "Finest" , "Furthest or maximum possible" , "Furthest point or degree of effort" .)