The most common and most likely answer to this crossword clue is the 8 letter word SIMULATE
I've seen this crossword clue in The Daily Telegraph
S
I
M
U
L
A
T
E
More clues leading to the result SIMULATE
- Pretend to be prompt, taking time out
- Assume I'm initially unknown, among list of candidates
- 19 Arouse with no time to reproduce
- Pretend to encourage having wasted time
- One Greek character in bluish- grey is mimic
- Encourage theatre leader to go and act
- Criminal must lie, covering up a sham
- Amulet is sadly fake
- Feign upset at 9
- Copy one Greek letter inscribed in tile
Buddy explains!

The straight part
We know that "act" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case act = simulate.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.
What does SIMULATE mean?
SIMULATE verb- To model, replicate, duplicate the behavior, appearance or properties of.
- Feigned; pretended.