Start with SUB and add the word described by the first clue to get the word described by the second clue.
-1. SUB + posture = Matter. 9 letters
-2. SUB + a seaman = Underwater. 9
-3. SUB + to message someone using a cellphone = Nuance or undertone. 7
-4. SUB + quest = The act of yielding to another. 10
-5. SUB + a piece of land = To deduct. 8
6. SUB + to combine = To go under the surface. 8
-7. SUB + a letter = Docile. 10
-8. SUB + edge = To sink. 7
-9. SUB + a citrus fruit = Transcendent. 7
-10. SUB + harden = Part of a bigger group. 6
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witter
I knew "missive" didn't seem quite right!