Highly formal attire that's used outside work (3,3)
I believe the answer is:
top hat
'highly formal attire' is the definition.
'top hat' can be an answer for 'attire' (I have seen 'Formal attire' mean 'top hat' so perhaps 'attire' could also mean 'top hat'). I am not sure about the 'highly formal' bit.
'used outside work' is the wordplay.
'used outside' becomes 'top' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'work' becomes 'hat' (hat is a kind of work).
'top'+'hat'='TOP-HAT'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for top hat that I've seen before include "Example of elegance" , "Headgear (for those in tails?)" , "Formal headgear for man" , "Wear it with tails" , "Formal headgear of lofty sort" .)