Left luggage outside, requiring extra special forms (7)
I believe the answer is:
classes
'forms' is the definition.
('class' can be a synonym of 'form')
'left luggage outside requiring extra special' is the wordplay.
I cannot really see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'special' could be 's' (common abbreviation as in Special Air Service) and 's' is present in the answer.
'luggage' could be 'cases' (I've seen this before) and 'cases' is present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for classes that I've seen before include "See 1" , "Groups with the same social or economic status" , "Categories" .)