The most common and most likely answer to this crossword clue is the 13 letter word CORRESPONDING
I've seen this crossword clue in The Daily Telegraph
C
O
R
R
E
S
P
O
N
D
I
N
G
Buddy explains!
The straight part
We know that "Relative/sending a postcard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relative/sending a postcard = corresponding.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 CORRESPONDING mean?
CORRESPONDING verb- (constructed with to) To be equivalent or similar in character, quantity, quality, origin, structure, function etc.
- (constructed with with) to exchange messages, especially by postal letter, over a period of time.
- To have sex with.
- A correspondence; the situation where things correspond or match.
- That have a similar relationship