Are they beastly at parties? (5)
I believe the answer is:
stags
This might be a double definition.
'are they' is the first definition.
The definition suggests a plural noun which matches the answer.
'beastly at parties?' is the second definition.
The answer and definition can be both to do with social activities as well as being verbs in their -s form.
Perhaps they are linked in a way I don't understand?
Can you help me to learn more?
(Another definition for stags that I've seen is " Male deer".)