Pointed remark on island prison and tower (8)
I believe the answer is:
barbican
'tower' is the definition.
(I know that Barbican is a type of tower)
'pointed remark on island prison' is the wordplay.
'pointed remark' becomes 'barb' (I've seen this before).
'on' means one lot of letters go next to another.
'island' becomes 'i' (geographical abbreviation).
'prison' becomes 'can' (the can is American slang for prison).
'barb'+'i'+'can'='BARBICAN'
'and' acts as a link.
(Other definitions for barbican that I've seen before include "Defensive tower above fortified city gate" , "Fortified structure" , "Outer defence of a castle or walled city" , "form of defence" , "London buildings with Arts Centre" .)