Body part causes irritation when undressed (5)
I believe the answer is:
ankle
'body part' is the definition.
(ankle is a kind of body part)
'causes irritation when undressed' is the wordplay.
'causes irritation' becomes 'rankles'.
'when undressed' means to look at the middle letters (remove the outside letters which 'dress' the word).
The central letters of 'rankles' are 'ANKLE'.
(Other definitions for ankle that I've seen before include "Part of the leg between the foot and the shin" , "Location of the talus" , "Narrow part of leg" , "Foot-leg connector" , "Human joint" .)