Density symbol, in physics (3)
I believe the answer is:
rho
'density symbol in physics' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rho that I've seen before include "Sophocles character" , "Something like 25 [ETA]" , "17th Greek letter" , "Seventeenth letter of the Greek alphabet" , "Greek character" .)