Doubtful strength? (5)
I believe the answer is:
might
This clue is a double definition.
'doubtful' is the first definition.
(I've seen this before)
'strength?' is the second definition.
(I know that might is a type of strength)
(Other definitions for might that I've seen before include "Powerfulness" , "Doubtful" , "Force; could" , "Strength - could possibly" , "Great power, strength" .)