The most common and most likely answer to this crossword clue is the 7 letter word ACRASIA
A
C
R
A
S
I
A
Buddy explains!

The straight part
We know that "Lack of will power" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lack of will power = acrasia.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.
Crossword Answer definitions
ACRASIA noun- Lack of self-control; excess; intemperance