Resolve then to do 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)
'resolve then to do' is the wordplay.
'resolve' indicates an anagram (I've seen 'resolving' mean this).
'then'+'to'+'do'='thentodo'
'thentodo' is an anagram of 'ON THE DOT'.
(Other definitions for on the dot that I've seen before include "Sharp" , "Absolutely punctual" , "on precise place in line?" , "at exactly the arranged time" , "Precisely punctual" .)