The most common and most likely answer to this crossword clue is the 8 letter word BIGHEADS
I've seen this crossword clue in The Times
B
I
G
H
E
A
D
S
More clues leading to the result BIGHEADS
Buddy explains!
The straight part
We know that "They are all I am" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they are all i am = bigheads.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 BIGHEADS mean?
BIGHEADS noun- (especially used by children) A person having an inflated opinion of himself; a conceited or arrogant person.
- One of several species of fish having a large head.
- One of several animal diseases that cause swelling of the head.