Willing to defer loan by letter (10)
I believe the answer is:
submissive
'willing' is the definition.
(synonyms)
'defer loan by letter' is the wordplay.
'defer loan by' becomes 'sub' (I can't justify this - if you can you should give a lot more credence to this answer).
'letter' becomes 'missive' (synonyms).
'sub'+'missive'='SUBMISSIVE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for submissive that I've seen before include "tendering resignation?" , "Meekly obedient" .)