The most common and most likely answer to this crossword clue is the 7 letter word REGNANT
I've seen this crossword clue in The Guardian
R
E
G
N
A
N
T
More clues leading to the result REGNANT
- Ruling expecting a child to doff its cap
- Now powerful outrage upset conservationists
- Head off meaningful ruling
- Scratching head, expecting ruling
- Ruling king’s son leaves with child
- On the throne, expecting to lose head
- Expecting to lose power as monarch
- Expecting to relinquish power for ruling
- Heading off with child leading
- Expecting to start late exercising power
Buddy explains!

The straight part
We know that "Now powerful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case now powerful = regnant.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 REGNANT mean?
REGNANT noun- A sovereign or ruler.
- Reigning, ruling; currently holding power.
- Dominant; holding sway; having particular power or influence.