Research establishment backing a Welsh lake (4)
I believe the answer is:
bala
'welsh lake' is the definition.
(Lake Bala is a Welsh lake)
'research establishment backing a' is the wordplay.
'research establishment' becomes 'lab' (laboratory).
'backing' says the letters should be written backwards.
'lab' written backwards gives 'bal'.
'bal'+'a'='BALA'
(Other definitions for bala that I've seen before include "Town and lake of North Wales" , "Largest natural lake in Wales" , "Welsh lake" .)