Medic at last probes painful scratch (5)
I believe the answer is:
score
'scratch' is the definition.
(score is a kind of scratch)
'medic at last probes painful' is the wordplay.
'at last' suggests the final letters.
'probes' indicates putting letters inside.
'painful' becomes 'sore' ('sore' can be a synonym of 'painful').
The last letter of 'medic' is 'c'.
'c' going into 'sore' is 'SCORE'.
(Other definitions for score that I've seen before include "Result" , "Tally; 20" , "Put ball in the net" , "record" , "Tally; group of twenty" .)