Keep quiet with yen to be diffident (3)
I believe the answer is:
shy
'diffident' is the definition.
(I know that diffident can be written as shy)
'keep quiet with yen' is the wordplay.
'keep quiet' becomes 'sh' (as in shushing someone).
'with' means one lot of letters go next to another.
'yen' becomes 'y' (resembles the symbol for the currency of Japan).
'sh'+'y'='SHY'
'to be' acts as a link.
(Other definitions for shy that I've seen before include "Timid (around people)" , "Tongue-tied" , "throw something at" , "Fling or throw" , "Reserved" .)