Joining in or amusing oneself away from the others (7,1,4)
I believe the answer is:
playing a part
'joining in' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs sometimes define each other.
'amusing oneself away from the others' is the wordplay.
'amusing oneself' becomes 'playing' ('play' can be a synonym of 'amuse oneself').
'away' becomes 'a' (abbreviation as used in sports fixtures).
'from' is a charade indicator (letters next to each other).
'the others' becomes 'part' (I can't justify this - if you can you should believe this answer much more).
'playing'+'a'+'part'='PLAYING A PART'
'or' acts as a link.
Can you help me to learn more?