Unclear second issue published additional books (15)

I believe the answer is:

semitransparent

I'm a little stuck... Click here to teach me more about this clue!

'unclear' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?

'second issue published additional books' is the wordplay.
'second' becomes 's' (common abbreviation).
'issue' becomes 'emit' (both can mean to give out or radiate).
'published' becomes 'ran' (as in a newspaper running a story).
'additional' becomes 'spare' (I've seen this before).
'books' becomes 'NT' (New Testament).
's'+'emit'+'ran'+'spare'+'nt'='SEMITRANSPARENT'

Can you help me to learn more?

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.