Big adjustable spanner a boon to estuary's shipping (5,6)
I believe the answer is:
tower bridge
'big adjustable spanner' is the definition.
(Tower Bridge spans the River Thames)
'a boon to estuary's shipping' is the wordplay.
I cannot quite understand how this works, but
'to' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tower bridge that I've seen before include "London landmark" , "sight in the capital" , "river crossing" , "a fixture between two Thames boroughs" .)