Quiet source of support I proffered (6)
I believe the answer is:
silent
'quiet' is the definition.
(similar in meaning)
'source of support i proffered' is the wordplay.
'source of' suggests taking the first letters (the start - or source - of the word).
'proffered' becomes 'lent' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'support' is 's'.
's'+'i'+'lent'='SILENT'
Can you help me to learn more?
(Other definitions for silent that I've seen before include "Unheard" , "Making no noise" , "Noiseless" , "Refusing to communicate" , "Like early films" .)