Harry Potter Munchkin: please help

[Hallieann22]

Sorry for the late reply. So I was in combat with a monster and was winning. My sister played a wandering monster with the LEVEL 4 COMMON WELSH GREEN. His card states (when it enters combat, your first task is to discard a card from your hand.)

I only had one card in my hand that was Engorgio: +10 to monster (if the monster is defeated we collect two extra treasures)

I wanted to just play this for the monster as I would have won even with the extra points to collect the extra treasures. However, my sister played the wandering monster with the monster that said I first needed to discard. So the question is can I play the card and then say I can’t discard because I don’t have a card orrrrr do I need to discard.