Dr. No returns carrying silver for legendary reptilian creature (6)
I believe the answer is:
dragon
'legendary reptilian creature' is the definition.
'dr no returns carrying silver' is the wordplay.
'returns' is a reversal indicator.
'carrying' says to put letters next to each other (in a down clue, the bottom letters carry others).
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'no' back-to-front is 'on'.
'on' after 'ag' is 'agon'.
'dr'+'agon'='DRAGON'
'for' acts as a link.
(Other definitions for dragon that I've seen before include "Mythical reptile" , "eg Puff, Fafner" , "St George's slew one" , "Mythical monster - large lizard" , "Intractable person" .)