Could do, you say, with a little (4)
I believe the answer is:
mite
'a little' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mite that I've seen before include "Tiny creature found in food" , "The widow's small offering" , "The widow's contribution, just in time" , "Minute arachnid" , "The widow's small donation" .)