Strengthen a delicate fabric (4)
I believe the answer is:
lace
I believe this clue is a double definition.
'strengthen' is the first definition.
(thesaurus)
'a delicate fabric' is the second definition.
(I've seen this in another clue)
(Other definitions for lace that I've seen before include "Spike - ornamental fabric" , "Famous play ''Arsenic and Old . . . .''" , "Delicate open-weave fabric" , "Do up (shoe, etc)" , "Delicate material" .)