Electrical activity starts beneath power plant (3)
I believe the answer is:
pea
'plant' is the definition.
(I know that pea is a type of bean)
'electrical activity starts beneath power' is the wordplay.
'starts' says to take the initial letters.
'beneath' is a charade indicator (letters next to each other) (in a down clue).
'power' becomes 'p' (symbol in physics).
The first letters of 'electrical activity' is 'ea'.
'ea' put after 'p' is 'PEA'.
(Other definitions for pea that I've seen before include "It grows in 23 Down" , "It may climb frame" , "Legume" , "Leguminous vegetable" , "Edible seed from pod" .)