Nuts caught in toilet (4)
I believe the answer is:
loco
'nuts' is the definition.
(synonyms)
'caught in toilet' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'in' indicates putting letters inside.
'toilet' becomes 'loo' (loo can informally mean a toilet).
'c' placed into 'loo' is 'LOCO'.
(Other definitions for loco that I've seen before include "Crazy in America - cool, man" , "plant increasing milk yield" , "Barmy (Spanish) - engine (abbr)" , "Transport" , "Slangily crazy" .)