Could be great power (5)
I believe the answer is:
might
I believe this is a double definition.
'could' is the first definition.
(synonyms)
'be great power' is the second definition.
(I've seen this before)
(Other definitions for might that I've seen before include "is not unlikely to" , "It expresses possibility" , "intensity of feeling" , "Great power, strength" , "word expressing possibility" .)