Strange nation, English, on reflection (5)
I believe the answer is:
eerie
'strange' is the definition.
(eerie means odd or strange)
'nation english on reflection' is the wordplay.
'nation' becomes 'Eire' (Eire is an example).
'english' becomes 'e' (abbreviation).
'on reflection' shows that the letters should be reversed in order.
'eire'+'e'='eiree'
'eiree' back-to-front is 'EERIE'.
(Other definitions for eerie that I've seen before include "spine-chilling" , "Scary, weird" , "Ghostly" , "Spine-tingling" , "Spooky, creepy" .)