Old prison ships (5)
I believe the answer is:
fleet
This clue is a double definition.
'old prison' is the first definition.
(Fleet Prison demolished in 1846)
'ships' is the second definition.
(I've seen this before)
(Other definitions for fleet that I've seen before include "good turn of speed" , "Naval force or collection of buses" , "Fast; London street, old prison" , "Fast kind of armada?" , "Fast - group of boats" .)