Schedule in heart unit (10)
I believe the answer is:
centimetre
'unit' is the definition.
(centimetre is a kind of unit)
'schedule in heart' is the wordplay.
'schedule' becomes 'time' (timing is a kind of scheduling).
'in' means one lot of letters goes inside another.
'heart' becomes 'centre' (synonyms).
'time' going inside 'centre' is 'CENTIMETRE'.
(Other definitions for centimetre that I've seen before include "From a little distance" , "About four tenths of an inch" , "Measure" , "Short length" , "Hundredth of a metre" .)