Boring person with new style, in retrospect (4)
I believe the answer is:
Here is my best explanation:
'boring person' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'with new style in retrospect' is the subsidiary indication.
'with' says to put letters next to each other.
'new' becomes 'n'.
'style' becomes 'way' ('way' can be a synonym of 'style'**).
'in retrospect' says the letters should be written backwards.
'n'+'way' is 'nway'.
'nway' backwards is 'yawn'.
(Other definitions for yawn that I've seen before include "Tedious thing", "Bored sign", "Expression of tiredness", "Mouth-stretching", "Oscitate", "Show boredom", "Sign of boredom", "Sign of tiredness", "Boring event" and "Show tiredness/boredom".)