The most common and most likely answer to this crossword clue is the 10 plus 4 letter word STATIONARY BIKE
I've seen this crossword clue in Financial Times
S
T
A
T
I
O
N
A
R
Y
B
I
K
E
Buddy explains!
The straight part
We know that "Taking 5 on this gets you nowhere!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case taking 5 on this gets you nowhere! = stationary bike.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.