Doing endless exercises, fast and slow (8)
I believe the answer is:
indolent
'slow' is the definition.
(thesaurus)
'doing endless exercises fast' is the wordplay.
'endless' means to remove the last letter.
'exercises' indicates an anagram (I've seen 'exercising' mean this).
'fast' becomes 'lent' (I've seen this before).
'doing' with its final letter taken away is 'doin'.
'doin' is an anagram of 'indo'.
'indo'+'lent'='INDOLENT'
'and' is the link.
(Other definitions for indolent that I've seen before include "Work-shy" , "Lethargic" , "Disinclined to exertion or work" , "Lazy, disinclined to work" , "Disinclined to exert oneself" .)