Fish river area, visiting West Sussex regularly (6)
I believe the answer is:
wrasse
'fish' is the definition.
(wrasse is a kind of fish)
'river area visiting west sussex regularly' is the wordplay.
'river' becomes 'r'.
'area' becomes 'a' (maths abbreviation).
'visiting' means one lot of letters goes inside another.
'west' becomes 'W' (synonyms).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'sussex' are 'sse'.
'r'+'a'='ra'
'w'+'sse'='wsse'
'ra' placed into 'wsse' is 'WRASSE'.
(Other definitions for wrasse that I've seen before include "Brightly-coloured fish" , "Type of fish" , "Bony fish" , "catch in the deep" , "Swears about the tropical marine fish with strong teeth" .)