Spiraea is also the name of the genus, hence plants.

The OTT clue was an edited version, I think I used ‘blue’ in the original.

Nimrod

As with him, I got [HIS]TORIOGRAP[HERS] quite easily, and [POST]E REST[ANTE] soon after. Then came [GO]B[STOP]PER, which meant that the deletions weren’t always leading and trailing letters, followed by [UP]SIDE-[DOWN], followed by … a long pause. So I eye-balled a map of London for a district that would fit with MU?? in the bottom right corner – MUS[WELL] H[ILL] and just three to go.

Another pause, and I could see that the entries to thematics G & H were 17 characters, down from 27 characters in the answers, so, 10 letters to be removed. These had to be two two-letter words and two three-letter words & probably a two- and a three- in each case: IN & OUT, and ON & OFF suggested themselves. I decided to resort to electronic aids. Pattern-matching *R*B*O*T* yielded 636 possibilities (pfff!), and – yes! – RA[IN]BOW TR[OUT]; easier going with *R*B*A*T*I*C* (only 35 possibilities) and I saw enough to get to PROBATI[ON] [OFF]ICE (not in Chambers).

One to go: the S?N entry in the top row from an 11 letter answer. No joy, so I slept on it. And, as I drifted into wakefulness on Bank Holiday Monday morning, into my head popped SUN[DAY] [NIGHT] – and I thought “How appropriate.”

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I agree with Ken about OTT in 17d, and with slaney about banan(A) at 21d, where “fruit” is part of the wordplay and the definition is just “salesman”. With regard to thematic G, the anagram indicator is “involved”, and “brawl” is not in fact doing double duty.

I seem to have gone on a bit … sorry.