A question for more experienced cryptic constructors: My original idea for the SPHERES clue was "They're round hairdos finished with dreads, I hear" - so, the last letter in "hairdos" + "fears" sounded out = SPHERES. I'm wondering, is it allowed to use a homophone clue for part of a word (and not a full word or a whole multi-word answer)?
Partial homophones like that are definitely used, it's just a matter of preference/fairness as to whether/how you use them. I think I usually stick to cluing homophones that are real words to be safe (e.g. Son reportedly pairs with extras=S+PARES), but I also think your example feels pretty fair and fun — as usual, there's really no hard rules, we're all just figuring it out as we go!
nate’s answer pretty much covers it, but my personal view is to avoid using homophones to clue a non-word prt of an answer — the rare times I’ve come across it in a published cryptic I haven’t been a fan. When it’s a syllable in the answer e.g. clueing SAWDUST with /sore/+DUST it’s definitely fine, other cases I’d treat with caution, as nate’s said with S+/pairs/. But that’s just my take, ultimately it’s all up to personal taste/style! Great puzzle by the way, thanks!