First, I don't actually believe at the moment that Clinsesmith will be sentenced in December. It will likely be continued as Clinesmith's cooperation will not be completed yet. (Flynn's sentencing was extended for two years for instance.)
There are a couple of issues swirling around this case. Clinesmith defrauded or caused a SSA of the FBI to defraud a FISA court. Judge Boasberg is currently the Chief Judge of the FISA courts and has been directed to clean up the messes from when Judge Collyer was the Chief Judge. That could be a conflict of interest causing him to recuse himself, or either party could ask him to recuse himself. That subject was broached at the hearing today and both parties declined to demand his recusal. So that issue is now moot.
BUT, Judge Boasberg directly commented that even if DOJ recommends a light sentence of 0-6 months in jail, he has the authority to go beyond that, meaning up to five years under the statute. However, assuming the plea agreement has a duty to cooperate, DOJ may not be happy with Clinesmith's cooperation and recommend more time. So this is really still up in the air.
As a general rule, federal judges don't much like attorneys defrauding their courts and making a mockery of justice. The ex parte nature of FISA proceedings heightens the duty of the government lawyers being scrupulously honest. And that was the majority of Clinesmith's actual job. The argument can be made that the judge would be more inclined to hammer him with somewhere over a year but less than five years just to send a powerful message for the future.
Spitballing here because there are a lot of balls in the air but my gut tells he'll eventually get 6-18 months in prison, five years probation. Now if Clinesmith is instrumental in bagging some big name, the reward for that might be a vastly reduced sentence as sending a message to others to cooperate with Durham/Barr going forward.
If Trump loses, Clinesmith likely skates, though.
My .02.