Eg Welsh commander-in-chief welcoming English officer (6)
I believe the answer is:
celtic
'eg welsh' is the definition.
(welsh is a kind of Celtic)
'commander-in-chief welcoming english officer' is the wordplay.
'commander-in-chief' becomes 'cic'.
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'english' becomes 'e' (abbreviation).
'officer' becomes 'lt' (I've seen this in other clues**).
'e'+'lt'='elt'
'cic' placed around 'elt' is 'CELTIC'.
(Other definitions for celtic that I've seen before include "Language group containing 14 & 15 across" , "Scottish football team" , "The Irish are part of the race" , "Racial of Welsh, Irish etc." , "The race of ancient Gaels" .)