Ask too much of public - a sign of error (7)
I believe the answer is:
overtax
'ask too much of' is the definition.
(I've seen this in another clue)
'public a sign of error' is the wordplay.
'public' becomes 'overt' (both can mean well-known or unconcealed).
'sign of error' becomes 'X' (X can indicate an error).
'overt'+'a'+'x'='OVERTAX'
(Other definitions for overtax that I've seen before include "Press too hard" , "exhaust" , "Place too great a burden on, as Revenue officials do" , "Impose too much strain on" , "Put too much pressure on, like Revenue Commissioners" .)