Son dropping plug causes obstruction (9-5)

'obstruction' is the definition.
(I know that stumbling block is a type of obstacle)

'son dropping plug' is the wordplay.
I cannot quite see how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is located in the answer.
'dropping' could be 'tumbling' (tumbling is a kind of dropping) and 'tumbling' is found within the answer.
'plug' could be 'block' (I've seen this in another clue) and 'block' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

'causes' is the link.

Can you help me to learn more?