The most common and most likely answer to this crossword clue is the 7 letter word RENAMED
R
E
N
A
M
E
D
Buddy explains!
The straight part
We know that "as 31/2 was to become 35/28d/2" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as 31/2 was to become 35/28d/2 = renamed.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 RENAMED mean?
RENAMED verb- To give a new name to.