Move quickly to secure prison (8)
I believe the answer is:
dartmoor
'prison' is the definition.
(English prison)
'move quickly to secure' is the wordplay.
'move quickly' becomes 'd'art' (I've seen this in another clue).
'to secure' becomes 'moor' (mooring is a kind of securing).
'dart'+'moor'='DARTMOOR'
(Other definitions for dartmoor that I've seen before include "Can people spend a lot of time here" , "Upland region in Devon" , "National Park in Devon" , "Well-known prison in Devon" , "place of H [HORSE]" .)