Row endlessly about nothing (3)
I believe the answer is:
nil
'nothing' is the definition.
('nil' can be a synonym of 'nothing')
'row endlessly about' is the wordplay.
'row' becomes 'line' (I've seen this before).
'endlessly' means to remove the last letter.
'about' shows that the letters should be reversed in order.
'line' with its last letter taken off 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" .)