Measure decline in deer (6)
I believe the answer is:
dosage
'measure' is the definition.
(I know that dosage is a type of indefinite quantity)
'decline in deer' is the wordplay.
'decline' becomes 'sag' (sag can mean to decline or droop).
'in' indicates putting letters inside.
'deer' becomes 'doe' (female deer).
'sag' inserted inside 'doe' is 'DOSAGE'.
(Other definitions for dosage that I've seen before include "Quantity of medication" , "Measured amount of medicine to be taken" , "amount of 14 [WONDER DRUG], say" , "Provision of medicine in prescribed amounts" , "Administered amount of drug" .)