Rock formation, perhaps, dry, but without phosphorus (4)
I believe the answer is:
arch
'rock formation' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for arch that I've seen before include "Affectedly playful like curved structure" , "Masonry structure for spanning an opening" , "Curved part of stone bridge, perhaps" , "Affectedly playful like senior angel" , "Clever" .)