Press old engineers to show where metal comes from (4,3)
I believe the answer is:
iron ore
'where metal comes from' is the definition.
(an iron ore contains iron)
'press old engineers' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'engineers' becomes 're' (abbreviation for Royal Engineers).
'iron'+'o'+'re'='IRON-ORE'
'to show' acts as a link.
(Other definitions for iron ore that I've seen before include "Mineral yielding common metal" , "From which a particular metal may be extracted" , "Aggregate from which the metal Fe can be extracted" , "Rock from which a metal can be extracted" , "Nero or I (anag) -- from which a common metal comes" .)