Underground worker runs after dog (7)
I believe the answer is:
collier
'underground' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'worker runs after dog' is the wordplay.
'worker runs' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'after' is a charade indicator (letters next to each other).
'dog' becomes 'collie' (collie is a kind of dog).
'r' put after 'collie' is 'COLLIER'.
Can you help me to learn more?
(Other definitions for collier that I've seen before include "He works in a coal mine" , "Geordie Jack, commonly" , "Ship carrying coal" , "Cargo ship" , "Painter" .)