Mum fleeing ocean predator in old boat (3)
I believe the answer is:
ark
'old boat' is the definition.
'mum fleeing ocean predator' is the wordplay.
'mum' becomes 'sh' (both can mean to be quiet).
'fleeing' indicates named letters should be taken away.
'ocean predator' becomes 'shark' (I've seen this in another clue).
'shark' with 'sh' removed is 'ARK'.
'in' acts as a link.
(Other definitions for ark that I've seen before include "The earliest vessel?" , "Rescue boat" , "Rescue ship" , "Great biblical boat" , "-- of the Covenant" .)