Large containers a lad stowed in vehicles (7)
I believe the answer is:
carboys
'large containers a lad' is the definition.
I can't tell whether this defines the answer.
'stowed in vehicles' is the wordplay.
'stowed' becomes 'boy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'vehicles' becomes 'cars' (car is a kind of vehicle).
'boy' placed within 'cars' is 'CARBOYS'.
Can you help me to learn more?
(Another definition for carboys that I've seen is " Means to carry corrosive".)