It's always in poetry that is spine-chilling (5)
I believe the answer is:
eerie
'spine-chilling' is the definition.
(I've seen this before)
'it's always in poetry that is' is the wordplay.
'it's always in poetry' becomes 'eer' (poetic version of 'ever').
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'eer'+'ie'='EERIE'
(Other definitions for eerie that I've seen before include "Unnerving" , "Strange, unusual" , "Ghostly" , "Spooky, unearthly" , "Strange and disturbing" .)