The most common and most likely answer to this crossword clue is the 5 plus 6 letter word FORTH BRIDGE
I've seen this crossword clue in The Guardian
F
O
R
T
H
B
R
I
D
G
E
More clues leading to the result FORTH BRIDGE
Buddy explains!
The straight part
We know that "Spanner/Spanner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spanner/spanner = forth bridge.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.