Turn back slow runners between defensive lines (8)
I believe the answer is:
midfield
'turn back slow runners between defensive lines' is the definition.
I can't tell whether this defines the answer.
'turn back slow runners between' is the wordplay.
'turn back' says the letters should be written backwards.
'slow' becomes 'dim' (synonyms).
'runners between' becomes 'field' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dim' in reverse letter order is 'mid'.
'mid'+'field'='MIDFIELD'
Can you help me to learn more?
(Other definitions for midfield that I've seen before include "Centre of pitch (soccer)" , "Central football area" .)