Being made of Lithium and Iron (4)
I believe the answer is:
life
'being' is the definition.
(life is a kind of being)
'lithium and iron' is the wordplay.
'lithium' becomes 'li' ('Li' can be a synonym of 'lithium').
'and' says to put letters next to each other.
'iron' becomes 'fe' ('Fe' can be a synonym of 'iron').
'li'+'fe'='LIFE'
'made of' acts as a link.
(Other definitions for life that I've seen before include "Baffin oiler (anag) - 7 film" , "The longest sentence" , ""But an empty dream" (Longfellow) - "As tedious as a twice-told tale" (Shakespeare)" , "Actuarial estimate of our remaining years" , "Animation" .)