Measure supposed healer in heart (10)
I believe the answer is:
centimetre
'measure' is the definition.
(centimetre is a kind of measure)
'supposed healer in heart' is the wordplay.
'supposed healer' becomes 'time' (proverbially, 'time heals all wounds').
'in' means one lot of letters goes inside another.
'heart' becomes 'centre' (synonyms).
'time' inserted inside 'centre' is 'CENTIMETRE'.
(Other definitions for centimetre that I've seen before include "Hundredth of a metre" , "Short distance" , "not giving an inch then?" , "Measure" , "From a little distance" .)