Scottish group's contribution to Gaelic language (4)
I believe the answer is:
clan
'scottish group's' is the definition.
(family or group of families in Scotland)
'contribution to gaelic language' is the wordplay.
'contribution to' says the answer is hidden in the clue.
'CLAN' can be found hidden inside 'gaelic language'.
(Other definitions for clan that I've seen before include "Kinship group" , "Tribal family" , "Scots family grouping" , "Scots family group" , "Scottish tribe" .)