Endless exercising beneath one — what's the alternative? (8)
I believe the answer is:
idleness
'the alternative?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'endless exercising beneath one' is the wordplay.
'exercising' indicates anagramming the letters.
'beneath' means one lot of letters go next to another (in a down clue).
'one' becomes 'I' (Roman numeral).
'endless' is an anagram of 'dleness'.
'dleness' put after 'i' is 'IDLENESS'.
'what's' acts as a link.
(Other definitions for idleness that I've seen before include "I'm having nothing to do" , "State of inactivity" , "Inaction" , "unemployment" , "Inactivity, indolence" .)