27 D is thematic, one containing 4 or more letters- a longer word, definition being a port in Holland. The thematic bit is at the beginning.
23. Isle - Muck is an isle, island. 'Lady' is the superfluous word in the clue, these are mentioned in the preamble.
21. The answer's simply hidden in the clue.
I can't remember who Cells related to, sorry.
PS. It's usually better to give the clues, so that people without the paper can help - but this was very thematic.