Father set of octuplets — a load to be carried (7)
I believe the answer is:
freight
'a load to be carried' is the definition.
(goods carried by a freight vehicle)
'father set of octuplets' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'set of octuplets' becomes 'eight' (eight children born at the same time).
'fr'+'eight'='FREIGHT'
(Other definitions for freight that I've seen before include "Commercial transport" , "Goods in transit" , "Goods carries by large vehicle" , "Transport of bulk goods" , "Consignment" .)