Taken from under mat? It isn't rash (10)
I believe the answer is:
dermatitis
'rash' is the definition.
(thesaurus)
'taken from under mat? it isn't' is the wordplay.
'taken from' indicates a hidden word.
'DERMATITIS' is hidden within 'under mat it isnt'.
(Other definitions for dermatitis that I've seen before include "Skin inflammation" , "Skin ailment" , "Skin trouble" , "Met diarist (anag.)" .)