Leader in Guardian newspaper backing animals (4)
I believe the answer is:
gnus
'animals' is the definition.
(I've seen this before)
'leader in guardian newspaper backing' is the wordplay.
'leader in' says to take the initial letters.
'newspaper' becomes 'sun' (UK newspaper The Sun).
'backing' shows that the letters should be reversed in order.
The first letter of 'guardian' is 'g'.
'sun' reversed gives 'nus'.
'g'+'nus'='GNUS'
(Other definitions for gnus that I've seen before include "Snug (anag) -- wildebeest" , "Gun (anag.)" , "African runners" , "runners in Africa" , "South African antelopes" .)