Idiot in Bielsa's side (3)
I believe the answer is:
ass
'idiot' is the definition.
(I've seen this in another clue)
'in bielsa's side' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ASS' can be found hidden inside 'bielsas side'.
(Other definitions for ass that I've seen before include "Donkey - fool" , "Braying animal" , "Turkey" , "Nincompoop" , "Jennet" .)