See Mike with a lead boxing, showing confidence (6)
I believe the answer is:
aplomb
'confidence' is the definition.
(I've seen this before)
'see mike with a lead boxing' is the wordplay.
'see' becomes 'lo' (archaic exclamation meaning 'look!').
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'with' means one lot of letters go next to another.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'boxing' is an insertion indicator ('box' can mean contain, as in to put in a box).
'a'+'pb'='apb'
'lo'+'m'='lom'
'lom' placed within 'apb' is 'APLOMB'.
'showing' acts as a link.
(Other definitions for aplomb that I've seen before include "Self-confidence, assurance" , "Cool self-assurance" , "Self-possession, assurance" , "coolness is required" , "Self-satisfied dignity and confidence" .)