Married woman captures the heart of karaoke bar (4)
I believe the answer is:
mars
'bar' is the definition.
(chocolate bar)
'married woman captures the heart of karaoke' is the wordplay.
'married woman' becomes 'mrs' (Mrs is an example).
'captures' means one lot of letters goes inside another.
'the heart of' indicates the central letters.
The middle of 'karaoke' is 'a'.
'mrs' placed around 'a' is 'MARS'.
(Other definitions for mars that I've seen before include "War god - chocolate bar - spoils" , "Red planet and god of war" , "Holst's Bringer of War" , "Famous Texas mission" , "Roman war god; planet" .)