slow puncture

[Vultch]

Air is leaking out of the base of the tyre valve not the actual valve itself but where it comes out of the rim, is this fixable..

yours etc.. who knows nothing about tyres

[Ploddit]

Normally oxidisation round the valve base.

Clean it up with emery paper and a new valve should sort it, if its really bad some patch glue around valve but the next tyre fitter will hate you for sticking in the valve.

[Bazcam]

If its a solid/metal valve stem you can sometimes get away with nipping
up the locking nut, if its on the outside, but this can also make it worse, if its a rubber valve stem, it needs cleaning as previously said.
Wichever , its an easy fix for any tyrehouse.