Many longings are kept out of sight (6)
I believe the answer is:
caches
'many longings are kept out of sight' is the definition.
I can't tell whether this definition defines the answer.
'many longings are kept out of' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for c00).
'longings' becomes 'ches' (I can't explain this - if you can you should believe this answer much more).
'are' becomes 'a' (short for 'are', historical unit of measurement).
'kept' means one lot of letters goes inside another.
'out of' indicates putting letters inside (some letters on the outside of others).
'ches' placed around 'a' is 'cahes'.
'c' inserted into 'cahes' is 'CACHES'.
Can you help me to learn more?
(Other definitions for caches that I've seen before include "Hidden valuables or ammunition" , "Secret hoards" , "conceal them?" , "Secret stores" .)