Head of brigade entering a lounge on fire (6)
I believe the answer is:
ablaze
'on fire' is the definition.
('ablaze' can be a synonym of 'on fire')
'head of brigade entering a lounge' is the wordplay.
'head of' says to take the initial letters.
'entering' indicates putting letters inside.
'lounge' becomes 'laze' (I've seen this before).
The initial letter of 'brigade' is 'b'.
'a'+'laze'='alaze'
'b' put within 'alaze' is 'ABLAZE'.
(Other definitions for ablaze that I've seen before include "Flaming; on fire" , "Fiercely aflame" , "On fire, burning" , "Bright" , "being toasted?" .)