Yellow bird by coalmine's entrance (6)
I believe the answer is:
craven
'yellow' is the definition.
(both can mean cowardly)
'bird by coalmine's entrance' is the wordplay.
'bird' becomes 'raven' (raven is a kind of bird).
'by' means one lot of letters go next to another.
'coalmine's entrance' becomes 'c' (I can't explain this - if you can you should believe this answer much more).
'raven' put after 'c' is 'CRAVEN'.
Can you help me to learn more?
(Other definitions for craven that I've seen before include "Chicken-hearted" , "Cowardly and fearful" , "Lacking courage, cowardly" , "Without backbone" , "Contemptibly cowardly" .)