The most common and most likely answer to this crossword clue is the 3 plus 3 plus 6 plus 1 plus 5 letter word ALL THE WORLDS A STAGE
I've seen this crossword clue in The Times
A
L
L
T
H
E
W
O
R
L
D
S
A
S
T
A
G
E
Buddy explains!
The straight part
We know that "Indication we’ll have to act" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case indication we’ll have to act = all the worlds a stage.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.