Measure function on used heart (4)
I believe the answer is:
dose
'measure' is the definition.
(dose is a kind of measure)
'function on used heart' is the wordplay.
'function' becomes 'do' (I've seen this in other clues**).
'on' means one lot of letters go next to another.
'heart' indicates the central letters.
The middle letters of 'used' are 'se'.
'do'+'se'='DOSE'
(Other definitions for dose that I've seen before include "Medicinal quantity" , "Give drugs to (a person)" , "Measure of medicine" , "Medicinal amount" , "Measure of drug" .)