Metal nut found by a lieutenant (6)
I believe the answer is:
cobalt
'metal' is the definition.
(I know that cobalt is a type of metal)
'nut found by a lieutenant' is the wordplay.
'nut' becomes 'cob' (cob is a kind of nut).
'found by' says to put letters next to each other.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'cob'+'a'+'lt'='COBALT'
(Other definitions for cobalt that I've seen before include "Bright blue pigment" , "Metal giving its name to type of deep blue" , "Hard ferromagnetic element, Co" , "Deep shade of 16 dn" , "Metal element" .)