Pretty wife wearing shirt (4)
I believe the answer is:
twee
'pretty' is the definition.
(twee can mean cute or pretty)
'wife wearing shirt' is the wordplay.
'wife' becomes 'w' (abbreviation).
'wearing' means one lot of letters goes inside another.
'shirt' becomes 'tee shirt' (tee shirt is a kind of shirt).
'w' going into 'tee' is 'TWEE'.
(Other definitions for twee that I've seen before include "Excessively pretty" , "Overly cute" , "Affectedly pretty" , "Sentimentally pretty" , "Sickly sweet" .)