There's nothing in unilateral action (3)
I believe the answer is:
nil
'there's nothing' is the definition.
(nil means a score of zero in football)
'in unilateral action' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NIL' is hidden within 'unilateral action'.
(Other definitions for nil that I've seen before include "none" , "Don't despair - . . . desperandum" , "Nothing found in senility" , "Nothing, no score" , "There's nothing left" .)