At home, fancy veil is keeping bride's head impossible to see (9)
I believe the answer is:
invisible
'impossible to see' is the definition.
(I've seen this before)
'at home fancy veil is keeping bride's head' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'fancy' is an anagram indicator.
'keeping' is an insertion indicator (some letters kept inside).
'head' indicates taking the first letters.
The first letter of 'brides' is 'b'.
'veil'+'is'='veilis'
'veilis' anagrammed gives 'visile'.
'visile' placed around 'b' is 'visible'.
'in'+'visible'='INVISIBLE'
(Other definitions for invisible that I've seen before include "Imperceptible by the eye" , "Hidden - imperceptible" , "Blind" , "Out of sight" , "Not able to be seen" .)