Row endlessly about nothing (3)
I believe the answer is:
nil
'nothing' is the definition.
(eg nil is zero in football)
'row endlessly about' is the wordplay.
'row' becomes 'line' (as in a row of things).
'endlessly' means to remove the last letter.
'about' is a reversal indicator.
'line' with its last letter removed is 'lin'.
'lin' back-to-front is 'NIL'.
(Other definitions for nil that I've seen before include "Don't despair - . . . desperandum" , "Nothing - in Latin" , "Nothing, naught" , "Nothing found in senility" , "0" .)