Idiot in Guardiola's side (3)
I believe the answer is:
ass
'idiot' is the definition.
(I've seen this in another clue)
'in guardiola's side' is the wordplay.
'in' indicates a hidden word.
'ASS' is hidden in the letters of 'guardiolas side'.
(Other definitions for ass that I've seen before include "Blockhead" , "Bottom, temporarily" , "Clinton's behind" , "mug" , "Animal related to horses" .)