Little boat's deviation from course line (4)
I believe the answer is:
yawl
'deviation from course line' is the definition.
I can't tell whether this defines the answer.
'little boat's' is the wordplay.
'little' means to remove the last letter (the word is not quite complete).
'boat's' becomes 'yawl's' (yawl is a kind of boat).
'yawls' with its last letter taken away is 'YAWL'.
Can you help me to learn more?
(Other definitions for yawl that I've seen before include "Sailing-ship" , "Two-masted sailing boat" , "Ships small boat" , "Two-masted sailing vessel" , "A two-master" .)