For the purposes of using 'spread', 'in combat' means that you've got some enmity from at least one enemy. If one of your party members enters combat, you'll have the combat music, but you won't have enmity from any of the critters until you either hit one with an offensive spell (like combust) or cast a healing type spell on a party member who already has enmity.

The game has a feature called the enmity list. It has green, yellow, orange, and red markers for any enemies that you've gotten enmity from. If anything is in this list, you should be able to use spread. If the list is empty, you will not be able to.