Bear caught in Welsh river flood (6)
I believe the answer is:
deluge
'flood' is the definition.
(I know that flood can be written as deluge)
'bear caught in welsh river' is the wordplay.
'bear' becomes 'lug' (both can mean to carry around).
'caught in' means one lot of letters goes inside another.
'welsh river' becomes 'dee' (I've seen this before).
'lug' going into 'dee' is 'DELUGE'.
(Other definitions for deluge that I've seen before include "Great flood" , "Inundation" , "Cloudburst, downpour" , "Swamp" , "A heavy downpour of rain" .)