Sea creature’s mournful cry picked up (5)
I believe the answer is:
whale
'sea creature's' is the definition.
(creature of the sea)
'mournful cry picked up' is the wordplay.
'mournful cry' becomes 'wail' (I've seen this in another clue).
'picked up' shows a homophone (sound like).
'wail' is a homophone of 'WHALE'.
(Other definitions for whale that I've seen before include "Mammoth marine mammal" , "Mammal like Moby Dick" , "Patch of oil, on water" , "Large marine creature" , "monster" .)