Do very well in old prison when cut off (5)
I believe the answer is:
excel
'do very well' is the definition.
(I've seen this before)
'old prison when cut off' is the wordplay.
'old' becomes 'ex'.
'prison' becomes 'cell' (I've seen this before).
'when cut off' means to remove the last letter.
'cell' with its last letter removed is 'cel'.
'ex'+'cel'='EXCEL'
'in' is the link.
(Other definitions for excel that I've seen before include "Perform outstandingly well" , "Be outstanding" , "Predominate in" , "Be highly proficient" , "surpass all others" .)