The most common and most likely answer to this crossword clue is the 7 plus 5 letter word IMPULSE BUYER
I've seen this crossword clue in The Times
I
M
P
U
L
S
E
B
U
Y
E
R
Buddy explains!
The straight part
We know that "I'll ignore spending plans" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i'll ignore spending plans = impulse buyer.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.