Like old Peruvian serving prison sentence (5)
I believe the answer is:
incan
'like old peruvian' is the definition.
(the Inca were a historical people of Peru)
'prison sentence' is the wordplay.
I cannot really see how this works, but
'prison' could be 'can' (the can is American slang for prison) and 'can' is present in the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'serving' is the link.
(Other definitions for incan that I've seen before include "doing time?" , "From old Peru" , "Pachacuti for example" , "See old South American" , "Of an ancient Andean culture" .)