Worst Irish cause to get moving (6)
I believe the answer is:
bestir
'to get moving' is the definition.
(I've seen this in another clue)
'worst irish' is the wordplay.
'worst' becomes 'best' (both can mean to beat someone).
'irish' becomes 'IR' (abbreviation).
'best'+'ir'='BESTIR'
'cause' acts as a link.
(Other definitions for bestir that I've seen before include "move" , "Exert/rouse (oneself)" , "Rouse (oneself)" , "Become active" .)