Surprisingly, Sam ain't showing endurance (7)
I believe the answer is:
stamina
'endurance' is the definition.
(I know that stamina is a type of endurance)
'surprisingly sam ain't' is the wordplay.
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
'sam'+'aint'='samaint'
'samaint' with letters rearranged gives 'STAMINA'.
'showing' acts as a link.
(Other definitions for stamina that I've seen before include "capability to keep going" , "Ability for sustained effort" , "Endurance, strength to stay the course" , "The ability to sustain prolonged physical or mental effort" , "This man owes his strength" .)