Go off on a tangent (7)
I believe the answer is:
digress
'go off on a tangent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for digress that I've seen before include "Turn aside (from the main subject?)" , "Get off the point" , "Go off the point" , "Ramble" , "Go off topic" .)