Prosthetic leg's easy to replace (5,3)
I believe the answer is:
glass eye
'prosthetic' is the definition.
(a glass eye is a prosthetic eye)
'leg's easy to replace' is the wordplay.
'to replace' indicates anagramming the letters (I've seen 'replaced' mean this).
'legs'+'easy'='legseasy'
'legseasy' is an anagram of 'GLASS EYE'.
(Other definitions for glass eye that I've seen before include "False socket filler" , "Organ substitute" , "Artificial organ" , "Easy legs (anag) - prosthetic device" , "no real looker?" .)