Power could — possibly? (5)
I believe the answer is:
might
I believe this clue is a double definition.
'power' is the first definition.
(I know that power can be written as might)
'could possibly?' is the second definition.
('could' can mean 'might')
(Other definitions for might that I've seen before include "is not unlikely to" , "Strength - could possibly" , "Great power/strength" , "Clout" , "What functions as an auxiliary" .)