Southern fish makes one shudder (5)
I believe the answer is:
shake
'shudder' is the definition.
(I know that shudder is a more specific form of the action tremble)
'southern fish' is the wordplay.
'southern' becomes 's' (abbreviation).
'fish' becomes 'hake' (hake is a kind of fish).
's'+'hake'='SHAKE'
'makes one' acts as a link.
(Other definitions for shake that I've seen before include "wave" , "Judder" , "Shiver" , "Rattle" , "Tremble, vibrate" .)