Mammals in ocean clash regularly (5)
I believe the answer is:
seals
'mammals' is the definition.
(I know this)
'ocean clash regularly' is the wordplay.
'ocean' becomes 'sea' (synonyms).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'clash' are 'ls'.
'sea'+'ls'='SEALS'
'in' is the link.
(Other definitions for seals that I've seen before include "Fastens or closes securely" , "Animals spotted on coast" , "they're impressive" , "They make an impression" , "Makes waterproof" .)