Shade during excavation nil (6)
I believe the answer is:
indigo
'shade' is the definition.
(shade of blue)
'during excavation nil' is the wordplay.
'during' becomes 'in' (I've seen this in other clues).
'excavation' becomes 'dig' (synonyms).
'nil' becomes 'o'.
'in'+'dig'+'o'='INDIGO'
(Other definitions for indigo that I've seen before include "Sixth rainbow colour" , "Deep violet-blue colour" , "Settlement; New Mexico native" , "Tropical plant" , "Colour - I'd go in (anag)" .)