The most common and most likely answer to this crossword clue is the 7 plus 4 letter word BOWLING BALL
I've seen this crossword clue in The Guardian
B
O
W
L
I
N
G
B
A
L
L
Buddy explains!
The straight part
We know that "One going down the lane /in front, swimmer having a good time (7,4)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one going down the lane /in front, swimmer having a good time (7,4) = bowling ball.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.