The most common and most likely answer to this crossword clue is the 6 letter word GIRDER
I've seen this crossword clue in The Daily Telegraph
G
I
R
D
E
R
Buddy explains!

The straight part
We know that "beam" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case beam = girder.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.
What does GIRDER mean?
GIRDER noun- A beam of steel, wood, or reinforced concrete, used as a main horizontal support in a building or structure.
- One who girds; a satirist.