Does order small prescribed portions of pills (5)
I believe the answer is:
doses
'prescribed portions of pills' is the definition.
'does order small' is the wordplay.
'order' is an anagram indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'does' is an anagram of 'dose'.
'dose'+'s'='DOSES'
(Other definitions for doses that I've seen before include "Gives medicine to" , "Potion portions" , "Prescribed amounts of medication" , "Administers medicine to (someone)" , "Medicine measures" .)