For foot one needs a doctor (4)
I believe the answer is:
iamb
'for foot' is the definition.
(type of metrical foot in poetry)
'one needs a doctor' is the wordplay.
'one' becomes 'i' (Roman numeral).
'needs' means one lot of letters go next to another.
'doctor' becomes 'MB' (Bachelor of Medicine).
'i'+'a'+'mb'='IAMB'
(Other definitions for iamb that I've seen before include "Short-long metrical foot" , "Metrical unit" , "Short-long metric foot" , "Line of verse" , "Metrical foot of one short, one long syllables" .)