Like CID drug incident under investigation (5-4)

I believe the answer is:

upper-case

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

'like cid drug' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'incident under investigation' is the wordplay.
'incident under' becomes 'upper' (I can't explain this - if you can you should believe this answer much more).
'investigation' becomes 'case'.
'upper'+'case'='UPPER-CASE'

Can you help me to learn more?

(Other definitions for upper-case that I've seen before include "Capital" , "In capitals" .)

I've seen this clue in The Times.

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