Letter from Greek character in steel production (7)
I believe the answer is:
epistle
'letter' is the definition.
(I know that letter can be written as epistle)
'greek character in steel production' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'in' means one lot of letters goes inside another.
'production' is an anagram indicator.
'steel' anagrammed gives 'estle'.
'pi' put into 'estle' is 'EPISTLE'.
'from' is the link.
(Other definitions for epistle that I've seen before include "Paul's work" , "Letter, in the Bible perhaps" , "Pets lie (anag)" , "Long formal letter (found in the New Testament?)" , "Missive; letter" .)