### Clue: Immature beasts ruin child's play (4,5)
### Answer: KIDS’ STUFF 
 ### Explanation: Definition: child's play
 KIDS(immature beasts, specifically young goats) + STUFF(informal term for alcohol or drugs – which could be one's ruin/something that ruins one's life, as in the old days when "gin was mother's ruin" – I guess).