Diminutive beasts of burden are wanting to turn back (5)
I believe the answer is:
small
'diminutive' is the definition.
(similar in meaning)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for small that I've seen before include "Unimportant; little" , "Not great (in number)" , "Piddling" , "Of no great size" , "Of little size" .)