Initially, Great Dane's upset when wife coming in gets bitten (6)
I believe the answer is:
gnawed
'bitten' is the definition.
(gnawing is a kind of biting)
'initially great dane's upset when wife coming in' is the wordplay.
'initially' says to take the initial letters.
'upset' shows that the letters should be reversed in order.
'when' becomes 'e' (I've seen this in other clues).
'wife coming' becomes 'wan' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
The first letters of 'great dane' is 'gd'.
'e'+'wan'='ewan'
'ewan' backwards is 'nawe'.
'gd' placed around 'nawe' is 'GNAWED'.
'gets' acts as a link.
(Other definitions for gnawed that I've seen before include "Bit persistently at something hard" , "Wore away; champed" , "Worried persistently" , "Chewed at like dog with bone" , "Persistently nibbled" .)