Iron and tungsten are rare (3)
I believe the answer is:
few
'rare' is the definition.
(thesaurus)
'iron and tungsten' is the wordplay.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'and' says to put letters next to each other.
'tungsten' becomes 'w' (W is the chemical symbol for tungsten).
'fe'+'w'='FEW'
'are' is the link.
(Other definitions for few that I've seen before include "Hardly anyone" , "Considerable number, ironically" , "Relatively small number" , "Not many, just a handful" , "Indefinite small amount" .)