Observe that it's just an excuse - they'll come to their senses (3,6)
I believe the answer is:
see reason
'just an excuse they'll come to their senses' is the definition.
I can't tell whether this definition defines the answer.
'observe that' is the wordplay.
'observe' becomes 'see' (observe can mean to see something).
'that' becomes 'reason' (I am not sure about this - if you are sure you should believe this answer much more).
'see'+'reason'='SEE REASON'
'it's' is the link.
Can you help me to learn more?
(Another definition for see reason that I've seen is " admit what makes sense".)