European region, one Lassie abandoned (7)
I believe the answer is:
silesia
'european region' is the definition.
(I've seen this before)
'one lassie abandoned' is the wordplay.
'one' becomes 'i' (Roman numeral).
'abandoned' is an anagram indicator.
'i'+'lassie'='ilassie'
'ilassie' with letters rearranged gives 'SILESIA'.
(Other definitions for silesia that I've seen before include "part of Europe" , "Region of central Europe" , "European region" , "Central European area" , "Polish area" .)