During excavation, opening for Oxford blue (6)
I believe the answer is:
indigo
'blue' is the definition.
(thesaurus)
'during excavation opening for oxford' is the wordplay.
'during' becomes 'in' (I've seen this in other clues).
'excavation' becomes 'dig' ('dig' can be a synonym of 'excavation').
'opening for' suggests taking the first letters.
The first letter of 'oxford' is 'o'.
'in'+'dig'+'o'='INDIGO'
(Other definitions for indigo that I've seen before include "supplier of blue stuff" , "Deep-blue dye or colour" , "Deep dark blue" , "type of dye" , "Settlement; New Mexico native" .)