Celtic language upset host by bash’s conclusion (5)
I believe the answer is:
welsh
'celtic language' is the definition.
(Welsh is an example)
'upset host by bash's conclusion' is the wordplay.
'upset' shows that the letters should be reversed in order.
'host' becomes 'slew' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'conclusion' suggests the final letters.
The final letter of 'bash' is 'h'.
'slew' in reverse letter order is 'wels'.
'wels'+'h'='WELSH'
(Other definitions for welsh that I've seen before include "So-called rabbit" , "- Dresser; - rarebit; - harp" , "From Swansea, say?" , "and 14 Down: Melted cheese on toast" , "Default on bet in Wales?" .)