It's always in poetry that is strange (5)
I believe the answer is:
eerie
'strange' is the definition.
(eerie means odd or strange)
'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' (abbreviation for id est, 'that is' in Latin).
'eer'+'ie'='EERIE'
(Other definitions for eerie that I've seen before include "Frighteningly odd" , "Spectral" , "Uncannily strange" , "Bloodcurdling" , "Spooky, weird" .)