In times gone by, prisoners drew on cell wall (4)
I believe the answer is:
once
'in times gone by' is the definition.
(I've seen this in another clue)
'prisoners drew on cell wall' is the wordplay.
I cannot quite see how this works, but
'on' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for once that I've seen before include "A single time" , "In former times" , "Formerly; as soon as" , "Cone (anag.)" , "Sometime" .)