Painful muscle contraction - runner's problem, primarily (5)
I believe the answer is:
cramp
'painful muscle contraction' is the definition.
(I've seen this before)
'runner's problem primarily' is the wordplay.
'runner' becomes 'cram' (Steve Cram, British runner).
'primarily' indicates taking the first letters.
The first letter of 'problem' is 'p'.
'cram'+'p'='CRAMP'
(Other definitions for cramp that I've seen before include "Sudden painful tightening of a muscle" , "Form of muscular pain" , "Muscle spasm" , "Give too little space to" , "Painful muscle problem" .)