The most common and most likely answer to this crossword clue is the 4 plus 7 plus 2 plus 5 letter word HAVE SOMEONE ON TOAST
I've seen this crossword clue in The Guardian
H
A
V
E
S
O
M
E
O
N
E
O
N
T
O
A
S
T
Buddy explains!
The straight part
We know that "Play totally dominant role" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case play totally dominant role = have someone on toast.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.