The most common and most likely answer to this crossword clue is the 15 letter word PENNY-FARTHINGS
I've seen this crossword clue in The Guardian
P
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F
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More clues leading to the result PENNY-FARTHINGS
Buddy explains!
The straight part
We know that "old bikes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old bikes = penny-farthings.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 PENNY-FARTHINGS mean?
PENNY-FARTHINGS noun- A type of bicycle popular in the 19th century, with the front wheel far larger than the rear one.