First stage of relay with a big noise being introduced (7)
I believe the answer is:
leading
'first' is the definition.
(I've seen this before)
'stage of relay with a big noise being introduced' is the wordplay.
'stage of relay' becomes 'leg'.
'with' is an insertion indicator.
'big noise' becomes 'din'.
'being introduced' means one lot of letters goes inside another.
'leg' placed around 'a' is 'leag'.
'leag' enclosing 'din' is 'LEADING'.
(Other definitions for leading that I've seen before include "Female top of the bill" , "Aligned in front" , "In the chair" , "Main" , "In pole position" .)