Second mate in a ship of the line (6)
I believe the answer is:
linear
'second mate in a ship of the line' is the definition.
I can't tell whether this defines the answer.
'second mate in a ship' is the wordplay.
'second mate' becomes 'a' (2nd letter of 'mate').
'in' is an insertion indicator.
'a ship' becomes 'liner' (as in an ocean liner).
'a' inserted into 'liner' is 'LINEAR'.
Can you help me to learn more?
(Other definitions for linear that I've seen before include "following straight path" , "In a straight line" , "(Of narrative) sequential" , "Arranged in straight fashion" , "adopting common narrative style" .)