Spin a coin (6)
(Other definitions for toss up that I've seen before include "close thing" , "It's as likely as not" , "Spin a coin - prepare food quickly" , "It's an even chance" , "unpredictable result" .)
'spin a coin' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tossup that I've seen before include "A gamble - flip a coin" , "Even chance or risk" , "Spin a coin - prepare food quickly" , "Situation with equally likely possible outcomes" , "Throw in air, coin or salad" .)