Separate account in heartless conclusion (6)
I believe the answer is:
detach
'separate' is the definition.
(detaching is a kind of separating)
'account in heartless conclusion' is the wordplay.
'account' becomes 'ac' (abbreviation).
'in' means one lot of letters goes inside another.
'heartless' says to hollow out the word (remove centre letters).
'conclusion' becomes 'death' (death is a kind of conclusion).
'death' with its centre taken out is 'deth'.
'ac' put into 'deth' is 'DETACH'.
(Other definitions for detach that I've seen before include "free" , "Remove, cut out" , "Disengage, unfasten" , "Separate one part from another" , "Remove from something else, say a coupon" .)