Go off course, say about two-fifths of a metre (5)
I believe the answer is:
stray
'go off course' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for stray that I've seen before include "Depart from the right path" , "Appearing occasionally" , "Wander off course" , "Drift - isolated" , "Deviate" .)