Cigarette smell in leather  

I used a leather cleaner and ...
I used a leather cleaner and leather protector from a company called furnitureclinic.co.uk. The suite I had utterly stank, so I went to it with first the cleaner, then the protector... I was pretty amazed by the results, so that's what I would use.
Stew

Just don't use anything
that contains petroleum distillates, alcohols, or dry-cleaning solvents.
Basically, if you wouldn't want it on your own skin, you don't want it on your leather. Limited-duration UV exposure, good ventilation, and time can help, too.
You can search "book conservation/preservation supplies" for vendors that sell specialized products such as cleaners and dressings used by museum conservators that most consider relatively safe.

First
I would just try airing it out for a while.

My Mobler couch
smelled but I simply used decent leather cleaner/conditioner on it and that pretty much did it. Since you should do that ANYWAY I would try that.

I purchased a lovely leather sofa for our living room with down filled cushions which was an absolute stinker.
I cleaned all the surfaces with a leather cleaner, the kitchen paper was yellow with nicotine.
I even threw in some odor eater insoles into the cushions and zipped them up, not sure how effective this was but time is a great healer....three months on the smell has almost dissipated.