Is it still libel if you don't name a particular person by his/her true name but
instead one uses a nickname that alludes to a particular person but doesn't
directly name them?
The answer is yes. If they are by any means identified then you may have well spelt out their name.
Many years ago, a writer published a story about a "John Smith" who was described in words that would cause offence.
A random John Smith stepped forward and claimed that the story impugned his good character and successfully sued for defamation. The story was not about this particular John Smith yet won his legal action and he collected damages.
Some years ago, an entire Year 12 at a Sydney high school took out a successful class action against a teacher and a newspaper who stated in print that they were a "bunch of hopeless losers" or words to that effect.
Beware even if you use somebody's initials! For example: If I said AZ from Burning Airlines was a #$%^& who *&^%$# gerbils and once did $%^&* with a melon and an Altar Boy, I could find myself in deep %^&* !
