Chain or whip, when not extremely penal (6)
I believe the answer is:
catena
'chain' is the definition.
(a chain or sequence of things)
'whip when not extremely penal' is the wordplay.
'whip' becomes 'cat' (cat o' nine tails, cat for short is a type of whip).
'when not extremely' says to take the centre (letters on the extremes of the word are removed).
The central letters of 'penal' are 'ena'.
'cat'+'ena'='CATENA'
'or' is the link.
(Other definitions for catena that I've seen before include "associated series to do with the bible" , "chain" .)