Pain caused by marathon and some needlework (7,6)
I believe the answer is:
running stitch
'some needlework' is the definition.
(running stitch is a kind of needlework)
'pain caused by' is the wordplay.
'pain' becomes 'stitch' (pain in the body).
'caused' becomes 'running' (I am not sure about this - if you are sure you should believe this answer much more).
'by' is a charade indicator (letters next to each other).
'stitch' after 'running' is 'RUNNING STITCH'.
'marathon and' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for running stitch that I've seen before include "continuous pain in side" , "Basic needlework style" , "Joiner" .)