I've used honey before with good results. Hopefully the crack isn't near the foot, If so it stands a good chance of continuing to split as the cigar burns.

On a side note, I had a RP Vintage '92 start spliting on me while I was smoking it. I pushed the bottom band down the cigar to the end of the split. The band kept the cigar from swelling as fast and stopped the split. It could have been a freak occurance but I'll try it again next time it happens.