Then do arrange, coming back, to be punctual (2,3,3)
I believe the answer is:
on the dot
'punctual' is the definition.
(I've seen this in another clue)
'then do arrange coming back to' is the wordplay.
'arrange' is an anagram indicator.
'coming back' is a reversal indicator.
'to' backwards is 'ot'.
'then'+'do'='thendo'
'thendo' is an anagram of 'onthed'.
'onthed'+'ot'='ON THE DOT'
'be' acts as a link.
(Other definitions for on the dot that I've seen before include "on precise place in line?" , "at exactly the arranged time" , "At exactly the right time" , "Prompt" , "Promptly" .)