Sustained energy at mains somehow (7)
I believe the answer is:
stamina
'sustained energy' is the definition.
(I've seen this before)
'at mains somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'at'+'mains'='atmains'
'atmains' with letters rearranged gives 'STAMINA'.
(Other definitions for stamina that I've seen before include "Enduring energy" , "Ability to maintain effort" , "This man owes his strength" , "toughness in > male parts" , "perseverance" .)