Rather thin, requiring millions invested in cure (6)
I believe the answer is:
remedy
'cure' is the definition.
(I know that cure can be written as remedy)
'rather thin requiring millions invested' is the wordplay.
I cannot really understand how this works, but
'millions' could be 'm' (abbreviation) and 'm' is located in the answer.
'thin' could be 'reedy' (as in a thin or reedy voice) and 'reedy' is found within the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for remedy that I've seen before include "Curative medicine" , "solution" , "Medicine or therapy that cures disease or relieves pain" , "Restorative" , "fix" .)