Ocean life regularly features this young animal (4)
I believe the answer is:
calf
'features this young animal' is the definition.
'calf' can be an answer for 'young' (calf is a kind of young). I'm not sure about the rest of the definition.
'ocean life regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'ocean'+'life'='oceanlife'
The alternating letters of 'oceanlife' are 'CALF'.
Can you help me to learn more?
(Other definitions for calf that I've seen before include "Baby cow" , "Young cow/bull" , "Muscular back of the shank" , "stocking filler" , "Young animal associated with soft leather" .)