Parts of plants sustained energy (7)
I believe the answer is:
stamina
This could be a double definition.
'parts of plants' is the first definition.
'stamina' can be an answer for 'parts' (I have seen 'Parts of flowers' mean 'stamina' so perhaps 'parts' could also mean 'stamina'). I'm not certain of the 'of plants' bit.
'sustained energy' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for stamina that I've seen before include "Ability to keep going" , "Staying-power, endurance" , "legs" , "perseverance" , "bottom" .)