Piece of headgear that is seen outside work (3,3)
I believe the answer is:
top hat
'piece of headgear' is the definition.
'top hat' can be an answer for 'headgear' (top hat is a kind of headgear). I'm not sure about the 'piece of' bit.
'that is seen outside work' is the wordplay.
'is seen outside' means one lot of letters goes inside another.
'work' becomes 'op' (abbreviation for opus).
'that' placed around 'op' is 'TOP-HAT'.
Can you help me to learn more?
(Other definitions for top hat that I've seen before include "Example of elegance" , "Formal head-wear for a man" , "It's seen with tails" , "Film with Astaire and Rogers" , "14 headgear" .)