They're always thin (7)
I believe the answer is:
veneers
'they're always thin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for veneers that I've seen before include "Thin wooden sheets" , "Coatings of a thin layers of superior quality" , "Superficial overlays" , "Fine wood facings" .)