Primly formal, yet somehow breezy? (5)

I believe the answer is:

stiff

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

'primly formal' is the definition.
'stiff' can be an answer for 'formal' (similar in meaning). I'm unsure of the 'primly' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for stiff that I've seen before include "Firm" , "Not easily bent" , "Rigid, unbending" , "Starched, rigid" , "Inflexible" .)

I've seen this clue in the Evening Standard.

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