Father about heartless performance: 'It's supposed to be funny' (6)
I believe the answer is:
satire
'it's supposed to be funny' is the definition.
(type of comedic writing or performance)
'father about heartless performance' is the wordplay.
'father' becomes 'sire' (synonyms).
'about' indicates putting letters inside.
'heartless' means to remove the middle letters.
'performance' becomes 'act' (act is a kind of performance).
'act' with its centre removed is 'at'.
'sire' enclosing 'at' is 'SATIRE'.
(Other definitions for satire that I've seen before include "Humorous criticism of folly or vice" , "Mockery" , "Parody, spoof" , "Writings ridiculing vice and folly" , "Witty language used to convey insults or scorn" .)