Noble gas identified by knight a long time in America (4)
I believe the answer is:
neon
'noble gas' is the definition.
(I've seen this before)
'knight a long time in america' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'a long time in america' becomes 'eon' (US spelling of 'aeon').
'n'+'eon'='NEON'
'identified by' acts as a link.
(Other definitions for neon that I've seen before include "Element that gives a red glow in a vacuum tube" , "Colourless noble gas" , "Noble gas used in illuminated signs" , "Small plant; colour" , "Gas used in fluorescent lighting" .)