Saint, tortured about a month, sustained energy (7)
I believe the answer is:
stamina
'sustained energy' is the definition.
(I've seen this before)
'saint tortured about a month' is the wordplay.
'tortured' indicates an anagram.
'about' means one lot of letters goes inside another.
'month' becomes 'M' (standard abbreviation).
'saint' anagrammed gives 'stina'.
'a'+'m'='am'
'stina' placed around 'am' is 'STAMINA'.
(Other definitions for stamina that I've seen before include "Parts of flowers" , "capability to keep going" , "Staying power, ability to withstand prolonged strain" , "fortitude" , "Ability to keep going" .)