Expired, as they're behind schedule (4)
I believe the answer is:
late
This could be a double definition.
'expired as' is the first definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives sometimes mean the same thing.
'behind schedule' is the second definition.
(running late)
'they're' acts as a link.
Can you help me to learn more?
(Other definitions for late that I've seen before include "after the sell-by date?" , "Still to come - already gone!" , "Dead or dilatory" , "Tardy, slow" , "In arrears" .)