The most common and most likely answer to this crossword clue is the 6 plus 5 letter word THAT’S RICH!
I've seen this crossword clue in The Guardian
T
H
A
T
â
€
™
S
R
I
C
H
!
Buddy explains!
The straight part
We know that "look who’s talking!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case look who’s talking! = that’s rich!.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.