Dilapidated facility fell, when river dropped (9)
I believe the answer is:
knackered
'dropped' is the definition.
Both the definition and answer are past participle verbs.
Maybe they are linked in a way I don't understand?
'dilapidated facility fell when river' is the wordplay.
'dilapidated facility fell' becomes 'knack' (I can't justify this - if you can you should give a lot more credence to this answer).
'when' becomes 'e' (I've seen this in other clues).
'river' becomes 'red' (Red River is an example).
'knack'+'e'+'red'='KNACKERED'
(Other definitions for knackered that I've seen before include "Exhausted (slang)" , "Worn out" .)