Having nothing, we must be in the red (3)
I believe the answer is:
owe
'be in the red' is the definition.
(I've seen this in another clue)
'having nothing we must' is the wordplay.
I cannot quite understand how this works, but
'nothing' could be 'o' (O resembles 0) and 'o' is found within the answer.
'we' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for owe that I've seen before include "Have to thank" , "Have outstanding" , "Ben in debt" , "Have obligation to pay" , "Be obliged to pay" .)