Being sensible when told what caused it (9,2,6)

'being sensible' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'when told what caused it' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?