The most common and most likely answer to this crossword clue is the 4 plus 3 plus 3 plus 5 letter word HOLD ALL THE CARDS
I've seen this crossword clue in The Guardian
H
O
L
D
A
L
L
T
H
E
C
A
R
D
S
Buddy explains!
The straight part
We know that "Be in full control" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be in full control = hold all the cards.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.