Could possibly show strength (5)
I believe the answer is:
might
I believe this is a double definition.
'could possibly' is the first definition.
('could' can mean 'might')
'strength' is the second definition.
(I know that might is a type of strength)
'show' acts as a link.
(Other definitions for might that I've seen before include "Clout" , "is not unlikely to" , "Powerfulness" , "Physical strength of old" , "It expresses possibility" .)