Head, copping it, taken aback, is to serve a prison sentence (2,4)
I believe the answer is:
do time
'to serve a prison sentence' is the definition.
(I've seen this before)
'head copping it taken aback' is the wordplay.
'head' becomes 'dome' (I've seen this before).
'copping' indicates putting letters inside (copping can mean receiving or accepting).
'taken aback' says the letters should be written in reverse.
'it' in reverse letter order is 'ti'.
'dome' enclosing 'ti' is 'DO TIME'.
'is' acts as a link.
(Other definitions for do time that I've seen before include "Serve porridge" , "Serve a prison sentence" .)