The most common and most likely answer to this crossword clue is the 7 letter word PIETISM
I've seen this crossword clue in The Times
P
I
E
T
I
S
M
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The straight part
We know that "excessive devotion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case excessive devotion = pietism.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 PIETISM mean?
PIETISM noun- (often capitalized) A movement in the Lutheran church in the 17th and 18th centuries, calling for a return to practical and devout Christianity.