Study, taking in facts about European rat (8)
I believe the answer is:
renegade
'rat' is the definition.
(thesaurus)
'study taking in facts about european' is the wordplay.
'study' becomes 'read' (synonyms).
'taking in' is an insertion indicator.
'facts' becomes 'gen'.
'about' shows that the letters should be reversed in order.
'european' becomes 'e' (abbreviation e.g. EU).
'gen' written backwards gives 'neg'.
'read' going around 'neg' is 'renegad'.
'renegad'+'e'='RENEGADE'
(Other definitions for renegade that I've seen before include "One who deserts a cause or group" , "Treacherous deserter" , "Apostate" , "Person who deserts from cause or faith" , "Treacherous person" .)