The most common and most likely answer to this crossword clue is the 4 letter word TROW
T
R
O
W
Buddy explains!

The straight part
We know that "Trust" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trust = trow.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 TROW mean?
TROW noun- Trust or faith.
- To trust or believe.
- To have confidence in, or to give credence to.
- Any of several flat-bottomed sailing boats used for fishing or for carrying bulk goods.
- Troll.