Sorry about writer and mock heartlessly (9)
I believe the answer is:
repentant
'sorry' is the definition.
(similar in meaning)
'about writer and mock heartlessly' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'writer' becomes 'pen' (something which writes).
'and' means one lot of letters go next to another.
'mock heartlessly' becomes 'tant' (I can't justify this - if you can you should believe this answer much more).
're'+'pen'+'tant'='REPENTANT'
Can you help me to learn more?
(Other definitions for repentant that I've seen before include "sorry state" , "Regretful" , "Contrite" , "showing contrition" , "Feeling remorse for misdeeds" .)