Photographs would be very useful - hard to say what's caused it without more information or seeing the tree. It might have an infection, the crack in the bark might be from damage, either from cold or some other cause, whether that crack is weeping or not is important, whether there are other areas which are soggy, etc.

The crack isn't weeping and tree is not in a soggy part of the garden.

Two causes for this kind of longitudinal cracking - hot days and cold nights, or alternating dry periods and very wet periods, essentially environmental conditions. These cracks may just callus over and the tree will be fine, or they may become infected over time and cause bleeding canker, but most trees with this problem live on for years. Just keep checking it periodically and if any weeping or sogginess develops, it should be taken down or removed.

Many thanks for your advice.