Arm possibly lacerated initially, one needing doctor (4)
I believe the answer is:
limb
'arm possibly' is the definition.
(arm is a kind of limb)
'lacerated initially one needing doctor' is the wordplay.
'initially' suggests taking the first letters.
'one' becomes 'i' (Roman numeral).
'needing' means one lot of letters go next to another.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
The initial letter of 'lacerated' is 'l'.
'l'+'i'+'mb'='LIMB'
(Other definitions for limb that I've seen before include "Eg, arm, leg" , "Part of body or branch of tree" , "Large tree branch" , "Tentacle" , "Branch - arm - leg" .)