Stop using a heartless stratagem? (4)
I believe the answer is:
stem
'stop' is the definition.
(I know that stem is a more specific form of the action check)
'a heartless stratagem?' is the wordplay.
'a heartless' says to hollow out the word (remove centre letters).
'stratagem' with its centre taken out is 'STEM'.
'using' is the link.
(Other definitions for stem that I've seen before include "Slender upright part of wine glass" , "part of a ship" , "Slender upright part, of flower or wineglass" , "Stalk - check" , "Staunch (flow)" .)