Runner covering extra length is a flour producer (6)
I believe the answer is:
miller
'a flour producer' is the definition.
(I've seen this before)
'runner covering extra length' is the wordplay.
I cannot quite understand how this works, but
'runner' could be 'r' (abbreviation) and 'r' is found within the answer.
'length' could be 'l' (abbreviation in geometry) and 'l' is located in the answer.
The remaining letters 'mile' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for miller that I've seen before include "He makes flour" , "Cereal worker" , "Producer of flour" , "he had a tale to tell!)" , "Grinder of flour" .)