Is it one who won't cross one's heart? (6)
I believe the answer is:
ticker
I believe this is a double definition.
'is it one who won't cross' is the first definition.
(someone giving ticks rather than crosses)
'one's heart?' is the second definition.
(ticker can informally mean the heart)
(Other definitions for ticker that I've seen before include "Maybe watch" , "Heart (sl.)" , "teacher giving correct marks" , "Slang heart or old spring watch" , "timer" .)