At exactly the right time, then, to do otherwise (2,3,3)
I believe the answer is:
on the dot
'at exactly the right time' is the definition.
(I've seen this before)
'then to do otherwise' is the wordplay.
'otherwise' is an anagram indicator.
'then'+'to'+'do'='thentodo'
'thentodo' is an anagram of 'ON THE DOT'.
(Other definitions for on the dot that I've seen before include "Promptly" , "Precisely punctual" , "Prompt" , "Absolutely punctual" , "at exactly the arranged time" .)