The most common and most likely answer to this crossword clue is the 7 letter word INFANTA
I've seen this crossword clue in The Times
I
N
F
A
N
T
A
More clues leading to the result INFANTA
- Admirer cheers following popular princess
- Trendy supporter cheers European royal
- Fashionable follower cheers Iberian princess
- Current admirer cheers the king of Spain's eldest daughter?
- Inferior worker meeting a king’s daughter
- Popular drink for a royal daughter?
- High-status woman elected, taking fizzy drink
- King’s daughter at home with drink
- Cool fizzy drink for princess
- Admirer cheers to support popular princess
Buddy explains!

The straight part
We know that "the king of Spain's eldest daughter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the king of spain's eldest daughter = infanta.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 INFANTA mean?
INFANTA noun- A daughter of a king in Spain and Portugal.