Mysterious European country sent up (5)
I believe the answer is:
eerie
'mysterious' is the definition.
(I've seen this in another clue)
'european country sent up' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'country' becomes 'eire' (Eire is an example).
'sent up' says the letters should be written in reverse.
'eire' reversed gives 'erie'.
'e'+'erie'='EERIE'
(Other definitions for eerie that I've seen before include "Weird, uncanny" , "Spooky, creepy" , "Unnerving" , "Sinister" , "Ghostly" .)