In a heartfelt way (6)
I believe the answer is:
dearly
'in a heartfelt way' is the definition.
(synonyms)
This is all the clue.
(Other definitions for dearly that I've seen before include "paying for big mistakes like this!" , "with much suffering" , "At a high price - with fondness" , "Very much; at great cost" , "Extremely - devotedly - at great cost" .)