Opening temperature seen in molten iron (5)
I believe the answer is:
intro
'opening' is the definition.
(short for introduction)
'temperature seen in molten iron' is the wordplay.
'temperature' becomes 't' (abbreviation).
'seen in' is an insertion indicator.
'molten' indicates anagramming the letters (I've seen this in other clues).
'iron' with letters rearranged gives 'inro'.
't' placed within 'inro' is 'INTRO'.
(Other definitions for intro that I've seen before include "preface" , "Beginning (informal)" , "Nitro (anag.)" , "Song's opening" , "Song's first few bars" .)