The most common and most likely answer to this crossword clue is the 4 plus 8 letter word SUIT YOURSELF
I've seen this crossword clue in The Times
S
U
I
T
Y
O
U
R
S
E
L
F
Buddy explains!
The straight part
We know that "Reject tailor's assistance/As you wish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reject tailor's assistance/as you wish = suit yourself.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.