You're already there -
You've got the word which acts as a suffix to give the definitions marsupials and tumblers in one group - that group is completed by another word (ie as defined) that can take the same suffix. The slight problem with the crossword is that the 3rd member of the group is defined without the suffix, while the definitions marsupials and tumblers include the suffix.
The second group comprises 3 words (as defined rather than as entered) which can take the other word you've found in the grid as a suffix. Chambers doesn't have the 1d definition-plus-suffix but it turns out to be a breakfast cereal.
The third group then take another suffix which means 'anagram' to people like us!