Truck that is bought to drop off heat-treated milk originally (5)
I believe the answer is:
bogie
'truck' is the definition.
(I know this)
'that is bought to drop off heat-treated milk originally' is the wordplay.
I cannot quite see how this works, but
'that is' could be 'ie' (short for Latin 'id est', 'that is') and 'ie' is located in the answer.
The remaining letters 'bog' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bogie that I've seen before include "Set of railcoach wheels" , "Freight truck" , "Wheeled undercarriage that supports a rail car" , "A four-wheeled truck" , "Part of carriage" .)