Key player in commercial having change of heart (5)
I believe the answer is:
primo
'key player' is the definition.
(leading part in a duet)
'in commercial having change of heart' is the wordplay.
'in commercial' becomes 'prim' (I am not sure about this - if you are sure you should believe this answer much more).
'having' is a charade indicator (letters next to each other).
'change' becomes 'bob' (I've seen this before).
'of heart' says to take the centre (I've seen 'heart' mean this).
The middle letter of 'bob' is 'o'.
'prim'+'o'='PRIMO'
(Other definitions for primo that I've seen before include "duettist" , "Principal part of duet" , "Right-hand part in piano duet" , "top player" , "Leading role" .)