Discovered metal-head - foremost creep (4)
I believe the answer is:
inch
'creep' is the definition.
(I've seen this before)
'discovered metal-head foremost' is the wordplay.
'discovered metal' becomes 'inc' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'foremost' says to take the initial letters.
The first letter of 'head' is 'h'.
'inc'+'h'='INCH'
Can you help me to learn more?
(Other definitions for inch that I've seen before include "2.54 centimetres" , "Islet (Scots); twenty-five.four mm" , "Move along gradually" , "Just a bit?" , "Scottish island: unit of length" .)