Work inside, ie with headgear (3,4)
I believe the answer is:
top hats
'work' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'inside ie with headgear' is the wordplay.
'inside' means to look at the middle letters.
'ie' becomes 'that is to say' (I've seen this before).
'with' says to put letters next to each other.
'headgear' becomes 'tophat' (top hat is a kind of headgear).
The central letter of 'thatistosay' is 's'.
's' after 'tophat' is 'TOP-HATS'.
Can you help me to learn more?
(Other definitions for top hats that I've seen before include "Sort of covers" , "formal gear" .)