Stray from a topic
I believe the answer is:
digress
'stray from a topic' 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 "Deviate from the main subject" , "Wander from one's path" , "Leave the main subject" , "Be sidetracked" , "Go off the main subject" .)