The most common and most likely answer to this crossword clue is the 8 letter word CELLMATE
I've seen this crossword clue in The Times
C
E
L
L
M
A
T
E
Buddy explains!
The straight part
We know that "You need time to get to know this fellow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case you need time to get to know this fellow = cellmate.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 CELLMATE mean?
CELLMATE noun- A person with whom one shares a prison cell.