Leave the main subject (7)
I believe the answer is:
digress
'leave the main subject' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for digress that I've seen before include "Leave the point" , "Go off topic" , "Stray from main subject" , "Turn aside (from the main subject?)" , "Deviate from the main subject" .)