Crossword Genius

Join (4,2)

Ross

'join' is the definition.
(I know that sign up is a more specific form of join)

This is the entire clue.

(Other definitions for sign up that I've seen before include "Commit to employment, education, etc" , "Take on" , "Enlist" , "Engage by written agreement" , "join the ranks" .)

I've seen this clue in The Wall Street Journal.
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