The most common and most likely answer to this crossword clue is the 5 plus 8 letter word WHITE ELEPHANT
I've seen this crossword clue in The Sunday Telegraph
W
H
I
T
E
E
L
E
P
H
A
N
T
More clues leading to the result WHITE ELEPHANT
- “Reinventing the wheel” inapt: it’s not needed
- Bank holiday, Spain — the plane’s heaving; it’s difficult to move
- I let His Excellency pawn the bizarre unwanted gift
- Help athlete win when running, avoiding one learner’s impediment
- Help a teen with ridiculously useless article
- Wheel patient being treated outside hospital — a useless and expensive thing
- Chess player’s big game that’s inconvenient but hard to shift
- First player on board: a trumpeter that’s not much use
- While a pet hen possibly needs time, this creature would be too extravagant a possession!
Buddy explains!
The straight part
We know that "Help a teen with" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case help a teen with = white elephant.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.