Positive response on European sight (4)
I believe the answer is:
eyes
'sight' is the definition.
(thesaurus)
'positive response on european' is the wordplay.
'positive response' becomes 'yes' (positive response to a question).
'on' is a charade indicator (letters next to each other).
'european' becomes 'E' (abbreviation e.g. EU).
'yes' after 'e' is 'EYES'.
(Other definitions for eyes that I've seen before include "Looks at - organs" , "Marks on potatoes" , "Potato seed-buds" , "Ophthalmologist's speciality" , "They have pupils" .)