Poetry virtually set in concrete — a change for the worse (8)
I believe the answer is:
reversal
'a change for the worse' is the definition.
(eg a reversal of fortunes)
'poetry virtually set in concrete' is the wordplay.
'poetry' becomes 'verse' (synonyms).
'virtually' means to remove the last letter (all but the last letter).
'set in' means one lot of letters goes inside another.
'concrete' becomes 'real' (associated in meaning).
'verse' with its final letter removed is 'vers'.
'vers' going inside 'real' is 'REVERSAL'.
(Other definitions for reversal that I've seen before include "A change to an opposite direction" , "Turnaround - defeat" , "About-turn" , "Change of fortune" , "U-turn" .)