clevis luks pass -d /dev/sda1 -s 1
clevis luks pass -d /dev/sda1 -s 1 <passphrase here>
Note that the output of clevis luks pass might be non-printable, in which case it would be better to redirect its output to a file and use it as a key file together with cryptsetup. For instance:
clevis luks pass -d /dev/sda1 -s 1 > slot1-passphrase
And the file slot1-passphrase will contain the passphrase associated with slot #1 in /dev/sda1.