Extra thin (5)
I believe the answer is:
spare
I believe this clue is a double definition.
'extra' is the first definition.
('spare' can be a synonym of 'extra')
'thin' is the second definition.
('spare' can be similar in meaning to 'thin')
(Other definitions for spare that I've seen before include "slit at top of trousers in Scotland" , "Extra; lean" , "This tyre is for emergencies" , "Extra item just in case" , "Refrain from using" .)