Irrational fifties remedy (4)
I believe the answer is:
pill
'remedy' is the definition.
(thesaurus)
'irrational fifties' is the wordplay.
'irrational' becomes 'pi' (an irrational number).
'fifties' becomes 'll' (Roman numerals**).
'pi'+'ll'='PILL'
(Other definitions for pill that I've seen before include "Medical tablet" , "Medication" , "Medicinal pellet" , "nuisance to put up with" , "Form of medicine" .)