Mostly heart, if I cease deviousness (8)
I believe the answer is:
artifice
'deviousness' is the definition.
(both can mean cunning or cleverness)
'mostly heart if i cease' is the wordplay.
'mostly' indicates a hidden word (most of the letters in the clue).
'ARTIFICE' can be found hidden inside 'heart if i cease'.
(Other definitions for artifice that I've seen before include "Trick, device" , "Clever trick, crafty ploy" , "Subtle deception" , "Clever trickery" , "Contrivance, trickery" .)