Gangrene initially appearing in undressed elbow (5)
I believe the answer is:
nudge
'elbow' is the definition.
('nudge' can be a synonym of 'elbow')
'gangrene initially appearing in undressed' is the wordplay.
'initially' says to take the initial letters.
'appearing in' indicates putting letters inside.
'undressed' becomes 'nude' (I've seen this before).
The first letter of 'gangrene' is 'g'.
'g' put inside 'nude' is 'NUDGE'.
(Other definitions for nudge that I've seen before include "Encouraging prompt" , "Festive entertainment" , "Jostle" , "---. ---, wink, wink (Monty Python)" , "Poke gently with elbow" .)