Article on board ship following a straight course (6)
I believe the answer is:
linear
'following a straight course' is the definition.
'article on board ship' is the wordplay.
'article' becomes 'a' (indefinite article).
'on board' means one lot of letters goes inside another.
'ship' becomes 'liner' (liner is a kind of ship).
'a' placed into 'liner' is 'LINEAR'.
(Other definitions for linear that I've seen before include "Of a line or length" , "In one dimension, I learn (anag.)" , "One-dimensional" , "following straight path" , "Straight; sequential" .)