One thousand stolen by word of agreement — “Oh, no!” (5)
I believe the answer is:
yikes
'agreement oh no' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'one thousand stolen by word' is the wordplay.
'one' becomes 'I' (Roman numeral).
'thousand' becomes 'K' (synonyms).
'stolen by' indicates putting letters inside.
'word' becomes 'yes' (I have seen 'Word of affirmation' mean 'yes' so perhaps 'word' could also mean 'yes').
'i'+'k'='ik'
'ik' put into 'yes' is 'YIKES'.
'of' is the link.
Can you help me to learn more?
(Other definitions for yikes that I've seen before include "Exclamation of surprise or alarm" , "alarmed expression" , "Gosh! - Oh no!" , "Expression of alarm or astonishment" , "Watch out" .)