Heartlessly pan bloomer — alternatively don't mention it! (2,7)
I believe the answer is:
no problem
'don't mention it' is the definition.
(phrases used when someone thanks you for something)
'heartlessly pan bloomer alternatively' is the wordplay.
'heartlessly' says to hollow out the word (remove centre letters).
'alternatively' is an anagram indicator.
'pan' with its middle taken out is 'pn'.
'pn'+'bloomer'='pnbloomer'
'pnbloomer' anagrammed gives 'NO PROBLEM'.
(Other definitions for no problem that I've seen before include "Easy peasy!" , "of course" , "fine" , "Don't worry about it!" .)