Person's worried prior to European reaction (8)
I believe the answer is:
response
'reaction' is the definition.
(synonyms)
'person's worried prior to european' is the wordplay.
'worried' indicates an anagram.
'prior to' says to put letters next to each other.
'european' becomes 'E' (abbreviation e.g. EU).
'persons' with letters rearranged gives 'respons'.
'respons'+'e'='RESPONSE'
(Other definitions for response that I've seen before include "Sees porn (anag) - answer" , "bid by partner" , "Reply, rejoinder" , "Answer - reaction" , "See porn (anag) -- answer" .)