Second mate in ship of the line (6)
I believe the answer is:
linear
'second mate in ship of the line' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'second mate in ship' is the wordplay.
'second mate' becomes 'a' (2nd letter of 'mate').
'in' indicates putting letters inside.
'ship' becomes 'liner' (liner is a kind of ship).
'a' inserted within 'liner' is 'LINEAR'.
Can you help me to learn more?
(Other definitions for linear that I've seen before include "Involving one dimension only" , "Long and narrow, of one dimension" , "Consisting of lines" , "(Of narrative) sequential" , "Arranged in straight fashion" .)