Periodic table item (7)
I believe the answer is:
element
'periodic table item' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for element that I've seen before include "eg Fire, air" , "Chemical building-block" , "iron or copper, perhaps" , "A first principle, found in a kettle?" , "Constituent part (7)" .)