description to come if answer is no for select field

[sasis]

I have a single select field with yes or no,I want users to enter a valid description if ansewr is no.Please advise

[Nic Brough -Adaptavist-]

Put the description field on screen, and then add a "validator" that says "if feild X has value Y, then require field Z to be filled as well"

You'll need a plugin to provide the validator, there isn't one that will do it off-the-shelf, but I'd go for the script-runner plugin and write one in there