Start to powerfully run, overcoming spasm in training (8)
I believe the answer is:
practice
'training' is the definition.
(practice is a kind of training)
'start to powerfully run overcoming spasm' is the wordplay.
'start to' suggests taking the first letters.
'run' becomes 'race' (synonyms).
'overcoming' means one lot of letters goes inside another.
'spasm' becomes 'tic' (tic is a kind of spasm).
The initial letter of 'powerfully' is 'p'.
'race' placed around 'tic' is 'ractice'.
'p'+'ractice'='PRACTICE'
'in' is the link.
(Other definitions for practice that I've seen before include "Learn by repetition" , "attempt at perfection" , "Habit - procedure" , "'....... what you preach'" , "Rehearsal" .)