Metal content of cage? (6)
I believe the answer is:
silver
'metal content of cage?' is the definition.
'silver' can be an answer for 'metal' (silver is a metallic element). I'm not certain of the 'content of cage?' bit.
'metal content of cage?' is the wordplay.
'metal' becomes 'V' (V is an example).
'content' means one lot of letters goes inside another.
'of cage?' becomes 'siler' (I can't justify this - if you can you should give a lot more credence to this answer).
'v' inserted inside 'siler' is 'SILVER'.
(Other definitions for silver that I've seen before include "Second-rank medal" , "old folk?" , "Twenty fifth anniversary metal" , "Soft, white, precious metal" , "Change - Ag" .)