The most common and most likely answer to this crossword clue is the 5 letter word BRICK
I've seen this crossword clue in The Daily Telegraph
B
R
I
C
K
Buddy explains!

The straight part
We know that "It’s helpful in building/supportive character" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s helpful in building/supportive character = brick.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 BRICK mean?
BRICK noun- A hardened rectangular block of mud, clay etc., used for building.
- Such hardened mud, clay, etc. considered collectively, as a building material.
- Something shaped like a brick.
- A helpful and reliable person.
- A shot which misses, particularly one which bounces directly out of the basket because of a too-flat trajectory, as if the ball were a heavier object.
- A power brick; an external power supply consisting of a small box with an integral male power plug and an attached electric cord terminating in another power plug.
- An electronic device, especially a heavy box-shaped one, that has become non-functional or obsolete.
- A carton of 500 rimfire cartridges, which forms the approximate size and shape of a brick.
- A community card (usually the turn or the river) which does not improve a player's hand.
- The colour brick red.
- One kilo of cocaine.
- To build with bricks.
- To make into bricks.
- To hit someone or something with a brick.
- To make an electronic device nonfunctional and usually beyond repair, essentially making it no more useful than a brick.
- (of weather) Extremely cold.