Power and energy beginning to affect plant (3)
I believe the answer is:
pea
'plant' is the definition.
(pea plant is a kind of plant)
'power and energy beginning to affect' is the wordplay.
'power' becomes 'p' (symbol in physics).
'and' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'beginning to' says to take the initial letters.
The initial letter of 'affect' is 'a'.
'p'+'e'+'a'='PEA'
(Other definitions for pea that I've seen before include "Ape this plant awkwardly" , "Climber" , "Pulse vegetable" , "Little green vegetable" , "It may climb frame" .)