Change course? Move erratically? Not entirely (4)
I believe the answer is:
veer
'change course?' is the definition.
(I've seen this before)
'move erratically? not entirely' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'VEER' is hidden within 'move erratically'.
(Other definitions for veer that I've seen before include "Swerve" , "Suddenly change direction" , "Change direction abruptly" , "Change direction or course, like wind" , "Deviate" .)