On and off, Tuncurry reverted to shrimp (4)
I believe the answer is:
runt
'shrimp' is the definition.
(I know that shrimp can be written as runt)
'on and off tuncurry reverted' is the wordplay.
'on and off' means one should take alternating letters (letters selected on and off at regular intervals).
'reverted' shows that the letters should be reversed in order.
The alternate letters of 'tuncurry' are 'tnur'.
'tnur' written backwards gives 'RUNT'.
'to' is the link.
(Other definitions for runt that I've seen before include "Weakling of the litter" , "Small pig" , "little piggy" , "Smallest pig in litter" , "little one" .)