Could be power (5)
I believe the answer is:
might
This clue is a double definition.
'could' is the first definition.
(synonyms)
'be power' is the second definition.
(I've seen this in another clue)
(Other definitions for might that I've seen before include "is not unlikely to" , "What functions as an auxiliary" , "word expressing possibility" , "Force; could" , "Strength - could possibly" .)