Of a group of languages such as Gaelic and Welsh (6)
I believe the answer is:
celtic
'of a group of languages such as gaelic and welsh' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for celtic that I've seen before include "Language group including Manx" , "The Sea between southern Ireland and Cornwall" , "Scottish club" , "Roy Keane's team on the fringe" , "Glasgow football team" .)