Half of pack trailing rugby scrum (4)
I believe the answer is:
ruck
'scrum' is the definition.
(I've seen this before)
'half of pack trailing rugby' is the wordplay.
'half of' indicates taking half.
'trailing' says to put letters next to each other.
'rugby' becomes 'ru'.
'pack' halved is 'ck'.
'ck' put after 'ru' is 'RUCK'.
(Other definitions for ruck that I've seen before include "Cower" , "Indistinguishable heap - a loose 10" , "Loose rugby scrum - the common crowd" , "Loose scrummage" , "Tightly packed crowd" .)