Click here for a link to a YouTube Channel Playlist containing videos explaining all the key skills you will need to bring from GCSE to A-Level chemistry.
Students who have studies Combined science are no less able to take A-level Chemistry than Triple students.
However, there is a certain amount of content which was not covered in the Combined Science GCSE syllabus which you will need to have an understanding of BEFORE starting A-Level chemistry.
As such it would be best for you to start off by working through the relevant sections of BBC Bitesize or Oak National Academy which cover these topics. Try to make good notes on the content and complete the quizzes and questions provided.