Some zinc over tin is concealed (6)
I believe the answer is:
covert
'is concealed' is the definition.
(similar in meaning)
'some zinc over tin' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'COVERT' is hidden in the letters of 'zinc over tin'.
(Other definitions for covert that I've seen before include "May hold the game" , "18 [HIDDEN]" , "Not acknowledged" , "Concealed like fox's liar" , "Hidden - thicket" .)