Lorry won't start, causing brawl (4)
I believe the answer is:
ruck
'brawl' is the definition.
'lorry won't start' is the wordplay.
'lorry' becomes 'truck' (I've seen this before).
'won't start' means to remove the first letter.
'truck' with its initial letter removed is 'RUCK'.
'causing' is the link.
(Other definitions for ruck that I've seen before include "Undistinguished crowd - irregular crease" , "Loose scrummage" , "Indistinguishable heap - a loose 10" , "Loose rugby scrum - the common crowd" , "A crease; loose scrum" .)