Building to do then, precisely (2,3,3)
I believe the answer is:
on the dot
'precisely' is the definition.
(on the dot can mean at a precise time)
'building to do then' is the wordplay.
'building' indicates anagramming the letters.
'to'+'do'+'then'='todothen'
'todothen' with letters rearranged gives 'ON THE DOT'.
(Other definitions for on the dot that I've seen before include "At exactly the right time" , "Promptly" , "Prompt" , "Precisely punctual" , "Sharp" .)