Mythical creature in loch captured by heartless city police (7)
I believe the answer is:
cyclops
'mythical creature' is the definition.
(I've seen this before)
'loch captured by heartless city police' is the wordplay.
'loch' becomes 'l'.
'captured by' means one lot of letters goes inside another.
'heartless' suggests removing the centre.
'police' becomes 'cops' (I've seen this before).
'city' with its middle taken out is 'cy'.
'cy'+'cops'='cycops'
'l' inserted within 'cycops' is 'CYCLOPS'.
'in' is the link.
(Other definitions for cyclops that I've seen before include "one with limited perception" , "The mythical giant with a single eye in the middle of his forehead" , "Mythical creature" , "antagonist of Ulysses" , "A one-eyed giant in Greek myth" .)