Streamlined fairing’s new, with excellent lines on plane’s tail (7)
I believe the answer is:
nacelle
'streamlined fairing's' is the definition.
I can't tell whether this definition defines the answer.
'new with excellent lines on plane's tail' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'with' is a charade indicator (letters next to each other).
'excellent' becomes 'ace' (ace can informally mean very good).
'lines' becomes 'll' (an abbreviation for line written twice).
'on plane's tail' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'n'+'ace'+'ll'+'e'='NACELLE'
Can you help me to learn more?
(Other definitions for nacelle that I've seen before include "airship's gondola" , "Enclosure for an aircraft engine" , "part of Jumbo" , "Streamlined housing on an aircraft" , "Streamlined cover for an aircraft engine" .)