Measures bit of salt eaten by deer (5)
I believe the answer is:
doses
'measures' is the definition.
(dose is a kind of measure)
'bit of salt eaten by deer' is the wordplay.
'bit of' suggests taking the first letters.
'eaten by' indicates putting letters inside.
'deer' becomes 'does' (I've seen this in other clues**).
The first letter of 'salt' is 's'.
's' inserted inside 'does' is 'DOSES'.
(Other definitions for doses that I've seen before include "Potion portions" , "Prescribed quantities of medicine" , "Prescribed amounts of medicine" , "Prescribed amounts of medication" , "Medicates" .)