Work to do, then, promptly (2,3,3)
I believe the answer is:
on the dot
'promptly' is the definition.
(on the dot means precisely at the appointed time)
'work to do then' is the wordplay.
'work' is an anagram indicator.
'to'+'do'+'then'='todothen'
'todothen' is an anagram of 'ON THE DOT'.
(Other definitions for on the dot that I've seen before include "Prompt" , "At precisely the right time" , "Precisely punctual" , "At exactly the right time" , "Promptly" .)