Tour ocean, regularly seeing killer whale (4)
I believe the answer is:
orca
'killer whale' is the definition.
(synonyms)
'tour ocean regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'tour'+'ocean'='tourocean'
The alternating letters of 'tourocean' are 'ORCA'.
'seeing' acts as a link.
(Other definitions for orca that I've seen before include "Marine creature" , "swimmer in black and white" , "grampus" , "School killer" , "Killer whales" .)