Interrupted only when I tend to digress (6,2)

'interrupted only' is the definition.
'butted in' can be an answer for 'interrupted' (butting in is a kind of interrupting). I am not sure about the 'only' bit.

'i tend to digress' is the wordplay.
I cannot really understand how this works, but
'i' is present in the answer.
an anagram of 'tend' is 'tedn' which is present in the left over letters.
The remaining letters 'but' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'when' is the link.

Can you help me to learn more?