Taking pains to enjoy oneself at great speed, covering in this way first of several miles (9)
I believe the answer is:
masochism
'taking pains to enjoy oneself at great speed covering in this way first of several miles' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'speed covering in this way first of several miles' is the wordplay.
'speed' becomes 'mach I' (Mach 1).
'covering' indicates putting letters inside.
'in this way' becomes 'so'.
'first of several' becomes 's' (1st letter of 'several').
'miles' becomes 'm' (common abbreviation).
'machi' enclosing 'so' is 'masochi'.
'masochi'+'s'+'m'='MASOCHISM'
Can you help me to learn more?
(Other definitions for masochism that I've seen before include "sex-based pleasure" , "Morbid gratification in suffering pain" , "Tendency to enjoy one's own suffering" .)