Let go without payment of even an old penny! (5)
I believe the answer is:
freed
'let go' is the definition.
(I've seen this in another clue)
'without payment of even an old penny' is the wordplay.
I cannot really understand how this works, but
'penny' could be 'd' (abbreviation e.g. Lsd - pounds, shillings, pence) and 'd' is present in the answer.
'without payment' could be 'free' (free of charge) and 'free' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for freed that I've seen before include "Untied" , "Emancipated, let loose" , "Defer being liberated" , "Released" .)