Collection of cruets knocked over containing the last of the oil (7)
I believe the answer is:
cluster
'collection' is the definition.
(cluster is a kind of collection)
'cruets knocked over containing the last of the oil' is the wordplay.
'knocked over' indicates anagramming the letters.
'containing' indicates putting letters inside.
'the last of' suggests the final letters.
The last letter of 'oil' is 'l'.
'cruets' with letters rearranged gives 'custer'.
'custer' going around 'l' is 'CLUSTER'.
'of' is the link.
(Other definitions for cluster that I've seen before include "Group or bunch" , "group together" , "Tight group of like things" , "Come together" , "Small group" .)