Could possibly be a synonym of power (5)
I believe the answer is:
might
'could' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for might that I've seen before include "Force; could" , "What functions as an auxiliary" , "- - and main" , "Clout" , "word expressing possibility" .)