Clue: Not quite slim and small, having eaten more than three biscuits (5,5)
Answer: PETIT FOURS
Explanation: Definition: biscuits
An envelope ('having eaten') of FOUR ('more than three') in PETIT[e] ('slim', usually applied to a woman, hence the feminine form) minus the last letter ('not quite') plus S ('small'). I would have called them small cakes rather than biscuits, but Chambers demurs.