They work and work around the clock (5)
I believe the answer is:
hands
'clock' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'they work and work around' is the wordplay.
'they work' becomes 'h' (I can't explain this - if you can you should believe this answer much more).
'work' becomes 'use' (work can mean to use or operate).
'around' indicates the central letters.
The central letter of 'use' is 's'.
'h'+'and'+'s'='HANDS'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for hands that I've seen before include "Players' cards" , "Body parts; crewmen" , "Crew members" , "Workmen" , "Gives" .)