Small letters put some luggage down (5,4)
I believe the answer is:
lower case
'small letters' is the definition.
(I've seen this before)
'some luggage down' is the wordplay.
I cannot really understand how this works, but
'some' could be 'a' and 'a' is found within the answer.
'down' could be 'lower' (synonyms) and 'lower' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'put' acts as a link.
Can you help me to learn more?
(Other definitions for lower case that I've seen before include "Small, UNLIKE THIS!" , "Unlike sentence, initially" , "perhaps it won't be heard in High Court?" , "Small letters" , "Minuscule" .)