Clear line in plot (5)
I believe the answer is:
plain
'clear' is the definition.
(similar in meaning)
'line in plot' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' indicates putting letters inside.
'plot' becomes 'pain' (I can't justify this - if you can you should give a lot more credence to this answer).
'l' going inside 'pain' is 'PLAIN'.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "Simply" , "no oil painting features" , "Unadorned on flat area" , "Undecorated; clear" , "Not decorated" .)