At heart, placidly sour (4)
I believe the answer is:
acid
'sour' is the definition.
(I've seen this before)
'at heart placidly' is the wordplay.
'at heart' indicates the answer is hidden within the clue (letters hidden in the heart of the clue).
'ACID' can be found hidden inside 'placidly'.
(Other definitions for acid that I've seen before include "Drug, tart" , "Acerbic" , "Sour, sharp-tasting" , "bad rain" , "Cutting" .)