Top removed from prison cells? That’s an error (4)
I believe the answer is:
oops
'that's an error' is the definition.
('oops' may be said after a mistake)
'top removed from prison cells?' is the wordplay.
'top removed from' means to remove the first letter.
'prison cells?' becomes 'coops' (coop is a type of prison or enclosure).
'coops' with its first letter taken away is 'OOPS'.
(Other definitions for oops that I've seen before include "Something's been spilt!" , "Exclamation of making a blunder" , "I've messed up" , "Clumsy me!" , "Exclamation recognising a mistake has been made" .)