The most common and most likely answer to this crossword clue is the 3 plus 10 plus 2 plus 5 plus 7 letter word THE IMPORTANCE OF BEING EARNEST
I've seen this crossword clue in Financial Times
T
H
E
I
M
P
O
R
T
A
N
C
E
O
F
B
E
I
N
G
E
A
R
N
E
S
T
Buddy explains!
The straight part
We know that "play" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case play = the importance of being earnest.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.