The most common and most likely answer to this crossword clue is the 2 plus 1 plus 3 plus 3 letter word BE A YES MAN
I've seen this crossword clue in Independent
B
E
A
Y
E
S
M
A
N
More clues leading to the result BE A YES MAN
Buddy explains!
The straight part
We know that "How to gain Trump’s favour?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how to gain trump’s favour? = be a yes man.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.