Positive response by European viewers (4)
I believe the answer is:
eyes
'viewers' is the definition.
'positive response by european' is the wordplay.
'positive response' becomes 'yes' (I've seen this before).
'by' means one lot of letters go next to another.
'european' becomes 'e' (abbreviation e.g. EU).
'yes' put after 'e' is 'EYES'.
(Other definitions for eyes that I've seen before include "Narrowly watches" , "front part of ship" , "Paradoxical title of 1999 Kubrick film" , "Scans" , "They have pupils" .)