Lead in truth, popular metal (3)
I believe the answer is:
tin
'metal' is the definition.
(tin is a kind of metal)
'lead in truth popular' is the wordplay.
'lead in' suggests taking the first letters.
'popular' becomes 'in'.
The initial letter of 'truth' is 't'.
't'+'in'='TIN'
(Other definitions for tin that I've seen before include "Silvery white metal" , "Silver-white element often used in food containers" , "Silvery-white metallic element" , "Metal element" , "loaf of bread" .)