Runner's problem with mounted frame (6,2)
I believe the answer is:
stitch up
'frame' is the definition.
(I've seen this before)
'runner's problem with mounted' is the wordplay.
'runner's problem' becomes 'stitch' (I can't justify this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'mounted' becomes 'up' (up on a horse).
'stitch'+'up'='STITCH-UP'
Can you help me to learn more?
(Other definitions for stitch up that I've seen before include "Incriminate by unfair means" , "frame" , "Fix rent" , "Falsely incriminate" , "Swindle - incriminate - repair" .)