Sometimes heartless form of address (5)
I believe the answer is:
madam
'sometimes heartless form of address' is the definition.
I can't tell whether this definition defines the answer.
'heartless form of address' is the wordplay.
'heartless' means to remove the middle letters.
'form' becomes 'maha' (I have seen 'Form of yoga ' mean 'maha' so perhaps 'form' could also mean 'maha').
'of address' becomes 'dam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'maha' with its middle removed is 'ma'.
'ma'+'dam'='MADAM'
Can you help me to learn more?
(Other definitions for madam that I've seen before include "precocious schoolgirl?" , "Formal address to a woman - precocious little girl" , "Woman of standing" , "Make wildly happy" , "Woman with status - girl with bad manners" .)