With nothing in it, a bag (4)
I believe the answer is:
nail
'bag' is the definition.
(thesaurus)
'with nothing in it a' is the wordplay.
I cannot quite understand how this works, but
'a' is found in the answer.
'nothing' could be 'nil' (eg nil is zero in football) and 'nil' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for nail that I've seen before include "stud" , "Hit this on the head to be bang-on" , "Bring down" , "Brad or tack" , "Hits this in the head to be bang-on" .)