The most common and most likely answer to this crossword clue is the 11 plus 6 letter word ABLE-BODIED SEAMEN
I've seen this crossword clue in The Times
A
B
L
E
-
B
O
D
I
E
D
S
E
A
M
E
N
Buddy explains!
The straight part
We know that "blokes on board" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case blokes on board = able-bodied seamen.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.