War paint certainly used in English borders — a form of woad (3,6)
I believe the answer is:
eye shadow
'war paint' is the definition.
'certainly used in english borders a form of woad' is the wordplay.
'certainly' becomes 'yes' (I've seen this before).
'used in' is an insertion indicator.
'borders' says to hollow out the word (remove centre letters) (only the edge or border letters remain).
'a form of' is an anagram indicator (the letters need to be in another 'form').
'english' with its middle taken out is 'eh'.
'woad' with letters rearranged gives 'adow'.
'eh'+'adow'='ehadow'
'yes' put inside 'ehadow' is 'EYE SHADOW'.
(Other definitions for eye shadow that I've seen before include "Coloured cosmetic" , "Facial cosmetic" , "Part of cosmetic treatment" , "Colouring cosmetic" , "application targeted at women" .)