Foot injury initially needing a doctor (4)
I believe the answer is:
iamb
'foot' is the definition.
(poetry rhythm)
'injury initially needing a doctor' is the wordplay.
'initially' suggests taking the first letters.
'needing' means one lot of letters go next to another.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
The first letter of 'injury' is 'i'.
'i'+'a'+'mb'='IAMB'
(Other definitions for iamb that I've seen before include "Metrical unit" , "Metrical foot of one short, one long syllables" , "Line of verse" , "Part of a pentameter?" , "Short-long metrical foot" .)