Raft beginning to sink in Welsh lake (5)
I believe the answer is:
balsa
'raft' is the definition.
(balsa raft is a kind of raft)
'beginning to sink in welsh lake' is the wordplay.
'beginning to' suggests taking the first letters.
'in' is an insertion indicator.
'welsh lake' becomes 'bala' (Lake Bala is a Welsh lake).
The first letter of 'sink' is 's'.
's' placed into 'bala' is 'BALSA'.
(Other definitions for balsa that I've seen before include "Tropical American tree" , "Very light timber" , "wood for raft" , "Strong lightweight wood" , "South American tree" .)