Twenty-four hours, but only half a second in time (6)
I believe the answer is:
monday
'twenty-four hours' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'only half a second in time' is the wordplay.
'only half' indicates taking half.
'in' means one lot of letters goes inside another.
'time' becomes 'may' (I've seen this before).
'second' halved is 'ond'.
'ond' put inside 'may' is 'MONDAY'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for monday that I've seen before include "May nod (anag.)" , "First weekday" , "Part of the week" , "Start of the working week" , "M in MTWTFSS" .)
