The historic prison in which we rot, imaginably (5)
I believe the answer is:
tower
'the historic prison' is the definition.
'tower' can be an answer for 'prison' (I have seen 'famous prison' mean 'tower' so perhaps 'prison' could also mean 'tower'). I'm unsure of the 'the historic' bit.
'which we rot imaginably' is the wordplay.
'we rot' anagrams to 'TOWER'.
But, I'm not sure how the anagram is indicated.
'in' is the link.
Can you help me to learn more?
(Other definitions for tower that I've seen before include "See 4 Down" , "famous prison" , "Steeple" , "Tall feature of Blackpool" , "Tall column" .)