Carry on with period (4)
I believe the answer is:
wage
'carry on' is the definition.
(as in waging war)
'with period' is the wordplay.
'with' becomes 'w' (abbreviation).
'period' becomes 'age' (age is a kind of period).
'w'+'age'='WAGE'
(Other definitions for wage that I've seen before include "Money earned" , "Engage in - pay" , "Money paid to worker" , "Pursue" , "Amount earned" .)