One with a penny debt might ... (5)
I believe the answer is:
power
'might' is the definition.
(being mighty)
'one with a penny debt' is the wordplay.
I cannot quite see how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'with' could be 'w' (abbreviation) and 'w' is found within the answer.
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is located in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for power that I've seen before include "See 12" , "Physical force, ability" , "Physical strength" , "- - of attorney" , "Large amount" .)