Mawkishness from son in returning embrace (4)
I believe the answer is:
gush
'mawkishness' is the definition.
'son in returning embrace' is the wordplay.
'son' becomes 's'.
'in' indicates putting letters inside.
'returning' says the letters should be written in reverse.
'embrace' becomes 'hug' ('hug' can be a synonym of 'embrace').
'hug' written backwards gives 'guh'.
's' going inside 'guh' is 'GUSH'.
'from' acts as a link.
(Other definitions for gush that I've seen before include "Talk effusively" , "Speak/write effusively" , "Be over-effusive" , "Spout like geyser" , "Speak effusively" .)