Flat out wearing a hat? (2,3,4)
I believe the answer is:
in top gear
'flat out' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'wearing a hat?' is the wordplay.
'wearing' becomes 'in' (as in being 'in' certain clothes).
'a hat?' becomes 'topgear' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in'+'topgear'='IN TOP GEAR'
Can you help me to learn more?
(Other definitions for in top gear that I've seen before include "driving fast?" , "Wearing trendy clothes" , "cruising" .)