Source of anxiety in retreating tiny animals (6)
I believe the answer is:
llamas
'animals' is the definition.
(I've seen this before)
'source of anxiety in retreating tiny' is the wordplay.
'source of' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'retreating' says the letters should be written backwards.
'tiny' becomes 'small' (similar in meaning).
The first letter of 'anxiety' is 'a'.
'small' reversed gives 'llams'.
'a' placed into 'llams' is 'LLAMAS'.
(Other definitions for llamas that I've seen before include "South American cud-chewing animals" , "South American animals related to camels but humpless" , "domesticated guanacos" , "South American animals like the camel" , "Andean pack animals" .)