Gas used in balloons (6)
I believe the answer is:
helium
'gas used in balloons' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for helium that I've seen before include "Atomic number 2" , "... ditto: symbol He" , "Light colourless gas used in balloons" , "Light colourless gas, sometimes in balloons" , "Floating balloon gas" .)