Plain clothes left one unsuspecting (9)

I believe the answer is:

oblivious

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

'unsuspecting' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I don't see?

'plain clothes left one' is the wordplay.
'plain' becomes 'obvious' (similar in meaning).
'clothes' indicates putting letters inside (some letters clothe or cover others).
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'l'+'i'='li'
'obvious' going around 'li' is 'OBLIVIOUS'.

Can you help me to learn more?

(Other definitions for oblivious that I've seen before include "Absent-mindedly unaware" , "In ignorance" , "Totally unaware" , "Apt to forget" , "like Lethe?" .)

I've seen this clue in The Daily Express.

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.