The most common and most likely answer to this crossword clue is the 5 letter word FELLA
I've seen this crossword clue in The Guardian
F
E
L
L
A
More clues leading to the result FELLA
Buddy explains!
The straight part
We know that "casual boyfriend" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case casual boyfriend = fella.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 FELLA mean?
FELLA noun- (chiefly South US) used to address a male
- A colleague or partner.
- A companion; a comrade.
- A man without good breeding or worth; an ignoble or mean man.
- An equal in power, rank, character, etc.
- One of a pair, or of two things used together or suited to each other; a mate.
- A person with common characteristics, being of the same kind, or in the same group.
- A male person; a man.
- A person; an individual, male or female.
- A rank or title in the professional world, usually given as "Fellow".
- (Aboriginal English) Used as a general intensifier
- A person who was a fellow attendee at one's school.