Correct line adopted by marketing (6)
I believe the answer is:
proper
'correct' is the definition.
(similar in meaning)
'line adopted by marketing' is the wordplay.
'line' becomes 'rope' (line can mean a piece of rope).
'adopted by' is an insertion indicator (I've seen this in other clues).
'marketing' becomes 'pr' (abbreviation for public relations).
'rope' placed into 'pr' is 'PROPER'.
(Other definitions for proper that I've seen before include "Appropriate according to rules" , "Fit; accurate; decent" , "Real; genuine" , "Seemly" , "name someone's given" .)