An agent brought up; building collapsed (8)
I believe the answer is:
perished
'collapsed' is the definition.
The answer and definition can be both related to change as well as being past participle verbs.
Perhaps there's an association between them I don't understand?
'an agent brought up building' is the wordplay.
'an' becomes 'i' (Roman numeral for one).
'agent' becomes 'rep' (short for representative).
'brought up' is a reversal indicator (down clues only).
'building' becomes 'shed' (shed is a kind of building).
'i'+'rep'='irep'
'irep' reversed gives 'peri'.
'peri'+'shed'='PERISHED'
Can you help me to learn more?
(Other definitions for perished that I've seen before include "Ruined" , "Decayed, died" , "(Of rubber) forty three" , "Died, failed" , "Extremely cold - died - rotted" .)