Two packs of aggregate including small credit (5)
I believe the answer is:
scrum
'two packs' is the definition.
(a rugby scrum is made up of a pack from each team)
'aggregate including small credit' is the wordplay.
'aggregate' becomes 'sum' (synonyms).
'including' is an insertion indicator.
'small credit' becomes 'cr' (abbreviation; 'small' suggests a short form).
'sum' placed around 'cr' is 'SCRUM'.
'of' acts as a link.
(Other definitions for scrum that I've seen before include "Jostling crowd" , "Feature of rugby match" , "disorderly group" , "Rugby get-together" , "Rugby formation - they get down to it" .)