The most common and most likely answer to this crossword clue is the 11 letter word CLOTHES-PIN
I've seen this crossword clue in The Times
C
L
O
T
H
E
S
-
P
I
N
Buddy explains!
The straight part
We know that "fix for costume" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fix for costume = clothes-pin.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.