Freight drivers' club is taken over by function (5)
I believe the answer is:
Here is my best explanation:
'freight' is the definition.
('cargo' can be a synonym of 'freight')
'drivers' club is taken over by function' is the wordplay.
'drivers' club' becomes 'rac' (UK automotive organisation).
'is taken over' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'function' becomes 'go' ('go' can be a synonym of 'function').
'rac' back-to-front is 'car'.
(Other definitions for cargo that I've seen before include "Hold contents", "Stuff on board", "Transported goods", "Payload", "Carried goods", "What gets carried", "Ship's freight", "Goods carried by ship, aircraft", "Goods carried on ship or plane" and "Carried load".)