One plus one is less than a dozen (6)
I believe the answer is:
eleven
'a dozen' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for eleven that I've seen before include "Football side" , "sports team?" , "Sporting side" , "Not first team (6,6) - See 24" , "'Legs' bingo number" .)