The most common and most likely answer to this crossword clue is the 7 letter word TRYSAIL
I've seen this crossword clue in Financial Times
T
R
Y
S
A
I
L
Buddy explains!

The straight part
We know that "item on the mainmast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case item on the mainmast = trysail.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 TRYSAIL mean?
TRYSAIL noun- A small, strong three-sided sail sometimes set in place of the mainsail in heavy weather.