Continue to block a medical treatment (7)
I believe the answer is:
plaster
'medical treatment' is the definition.
(a broken body part might be put in plaster)
'continue to block a' is the wordplay.
'continue' becomes 'last' (I've seen this before).
'to block' means one lot of letters goes inside another (block can mean to fill a hole in something).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'last' inserted into 'per' is 'PLASTER'.
(Other definitions for plaster that I've seen before include "Substance for coating walls" , "cake" , "Wall and ceiling covering" , "Smear" , "____ of Paris" .)