Donkey in Guardiola's side (3)
I believe the answer is:
ass
'donkey' is the definition.
(ass is a term for a donkey)
'in guardiola's side' is the wordplay.
'in' says the answer is hidden in the clue.
'ASS' is hidden in the letters of 'guardiolas side'.
(Other definitions for ass that I've seen before include "An American backside" , "Moke" , "Dipstick" , "Trump's behind" , "Hoofed animal, smaller than a horse" .)