The most common and most likely answer to this crossword clue is the 10 letter word UNDERWROTE
I've seen this crossword clue in The Daily Telegraph
U
N
D
E
R
W
R
O
T
E
Buddy explains!

The straight part
We know that "Became a subscriber" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case became a subscriber = underwrote.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 UNDERWROTE mean?
UNDERWROTE verb- To write below or under; subscribe.
- To subscribe (a document, policy etc.) with one's name.
- To sign; to put one's name to.
- To agree to pay by signing one's name; subscribe.
- Specifically, to assume financial responsibility for something, and guarantee it against failure.
- To act as an underwriter.
- To support, lend support to, guarantee the basis of.
- To submit to; put up with.