Phosphorus, funny medicine (4)
I believe the answer is:
pill
'medicine' is the definition.
(I know that pill is a type of dosage)
'phosphorus funny' is the wordplay.
'phosphorus' becomes 'P' (P is the chemical symbol for phosphorus).
'funny' becomes 'ill' (eg feeling a bit funny).
'p'+'ill'='PILL'
(Other definitions for pill that I've seen before include "Medicinal pellet" , "nuisance to put up with" , "Medicinal tablet" , "Medication" , "Medical tablet" .)