Heartily shower in what way? (3)
I believe the answer is:
how
'in what way?' is the definition.
(synonyms)
'heartily shower' is the wordplay.
'heartily' indicates the answer is hidden within the clue (letters at the heart of the clue).
'HOW' is hidden within 'shower'.
(Other definitions for how that I've seen before include "By what means?" , "Who (anag.)" , "In what way?" , "See 16" , "In what manner or method?" .)