Shrinks back in fear as animal starts exhibiting rare symptoms (6)
I believe the answer is:
cowers
'shrinks back in fear' is the definition.
(I've seen this before)
'animal starts exhibiting rare symptoms' is the wordplay.
'animal' becomes 'cow' (cow is a kind of animal).
'starts' indicates taking the first letters.
The first letters of 'exhibiting rare symptoms' is 'ers'.
'cow'+'ers'='COWERS'
'as' acts as a link.
(Other definitions for cowers that I've seen before include "Shrinks from escrow" , "Flinches, shrinks back in fear" , "Quails" , "Crouches down in fear" .)