What about this writer, wearing Lieutenant's headgear! (6)
I believe the answer is:
helmet
'headgear' is the definition.
(helmet is a kind of headgear)
'what about this writer wearing lieutenant's' is the wordplay.
'what' becomes 'eh' (I've seen this in other clues).
'about' says the letters should be written backwards.
'this writer' becomes 'me' (the person who is writing these clues).
'wearing' means one lot of letters goes inside another.
'lieutenant' becomes 'lt'.
'eh' reversed gives 'he'.
'me' inserted inside 'lt' is 'lmet'.
'he'+'lmet'='HELMET'
(Other definitions for helmet that I've seen before include "Protection for top" , "Protective casing for the head" , "to provide capital protection" , "Protective hat" , "Armour for the head" .)