Improve, retaining a lot of speed on a curved track (7)
I believe the answer is:
helical
'curved track' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs can occasionally define each other.
'improve retaining a lot of speed' is the wordplay.
'improve' becomes 'heal' (healing is a kind of improving).
'retaining' means one lot of letters goes inside another (inserted letters are held or retained).
'a lot of speed' becomes 'lic' (I can't justify this - if you can you should give a lot more credence to this answer).
'heal' enclosing 'lic' is 'HELICAL'.
'on a' acts as a link.
Can you help me to learn more?
(Other definitions for helical that I've seen before include "Like a spiral or spring" , "Spiral-shaped" , "Like the pattern on a snail's shell" , "Curled like corkscrew" , "In the form of a spiral" .)