Show wrong sign in European river (4)
I believe the answer is:
expo
'show' is the definition.
(I've seen this before)
'wrong sign in european river' is the wordplay.
'wrong sign' becomes 'x' (x is used to show an incorrect answer).
'in' means one lot of letters goes inside another.
'european' becomes 'e' (abbreviation e.g. EU).
'river' becomes 'po' (Italian river).
'e'+'po'='epo'
'x' placed into 'epo' is 'EXPO'.
(Other definitions for expo that I've seen before include "Large international fair" , "Major international fair" , "Public exhibition" , "Trade fair" , "Public spectacle" .)