The most common and most likely answer to this crossword clue is the 4 plus 3 plus 3 plus 3 plus 5 letter word PULL OUT ALL THE STOPS
I've seen this crossword clue in The Times
P
U
L
L
O
U
T
A
L
L
T
H
E
S
T
O
P
S
Buddy explains!
The straight part
We know that "Spare no effort" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spare no effort = pull out all the stops.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.