The most common and most likely answer to this crossword clue is the 7 plus 7 letter word BATHING COSTUME
I've seen this crossword clue in The Daily Telegraph
B
A
T
H
I
N
G
C
O
S
T
U
M
E
More clues leading to the result BATHING COSTUME
Buddy explains!
The straight part
We know that "Good to wear it on beach — must when swimming" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case good to wear it on beach — must when swimming = bathing costume.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.