Runs through — tries to stop Welshman? (9)
I believe the answer is:
rehearses
'runs through' is the definition.
(I've seen this in another clue)
'tries to stop welshman?' is the wordplay.
'tries' becomes 'hears' (as in trying or hearing in a trial).
'to stop' indicates putting letters inside (stopping can mean blocking a gap).
'welshman?' becomes 'rees' (common Welsh name).
'hears' placed inside 'rees' is 'REHEARSES'.
(Other definitions for rehearses that I've seen before include "not wearing dress till later?" , "Runs through" , "Practises as actors do" , "Practises a play" .)