Moment to enter heart unit (10)
I believe the answer is:
centimetre
'unit' is the definition.
(I know that centimetre is a Unit of length)
'moment to enter heart' is the wordplay.
'moment' becomes 'time' (time is a kind of moment).
'to enter' is an insertion indicator.
'heart' becomes 'centre' (synonyms).
'time' placed into 'centre' is 'CENTIMETRE'.
(Other definitions for centimetre that I've seen before include "Short distance" , "not giving an inch then?" , "Short length" , "Hundredth of a metre" , "Measure" .)