The most common and most likely answer to this crossword clue is the 7 letter word FEEBLER
I've seen this crossword clue in The Daily Telegraph
F
E
E
B
L
E
R
Buddy explains!

The straight part
We know that "Weaker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case weaker = feebler.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 FEEBLER mean?
FEEBLER adjective- Deficient in physical strength
- Lacking force, vigor, or efficiency in action or expression; faint.