Converted cabin behind pub in tower (8)
I believe the answer is:
barbican
'tower' is the definition.
(I've seen this before)
'converted cabin behind pub' is the wordplay.
'converted' is an anagram indicator.
'behind' says to put letters next to each other.
'pub' becomes 'bar' (I've seen this before).
'cabin' anagrammed gives 'bican'.
'bican' put after 'bar' is 'BARBICAN'.
'in' is the link.
(Other definitions for barbican that I've seen before include "Fortification's watchtower" , "London buildings with Arts Centre" , "Fortified structure" , "Fortified gateway" , "Fortified watchtower over a city gate" .)