Begin again on problem, last of three (6)
I believe the answer is:
resume
'begin again' is the definition.
(resume can mean to start doing something again)
'on problem last of three' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'problem' becomes 'sum' (arithmetic problem).
'last of' indicates one should take the final letters.
The last letter of 'three' is 'e'.
're'+'sum'+'e'='RESUME'
(Other definitions for resume that I've seen before include "Go on after a pause" , "Precis" , "Continue - summary" , "Carry on where one left off" , "Re-open" .)