Took in from undersea terrain (3)
I believe the answer is:
ate
'took in' is the definition.
(take in can mean to eat)
'from undersea terrain' is the wordplay.
'from' says the answer is hidden in the clue.
'ATE' can be found hidden inside 'undersea terrain'.
(Other definitions for ate that I've seen before include "Worried" , "Took food" , "Had at meal" , "Consumed food" , "mischief-maker" .)