Argue on and off for years? (3)
I believe the answer is:
age
'years?' is the definition.
(period of many years)
'argue on and off' is the wordplay.
'on and off' means one should take alternating letters (letters selected on and off at regular intervals).
The alternate letters of 'argue' are 'AGE'.
'for' acts as a link.
(Other definitions for age that I've seen before include "Years since birth" , "Era; grow old" , "Length of life" , "Time in years" , "Just a number" .)