Welshman's building a platform (4)
I believe the answer is:
dais
'a platform' is the definition.
(I know that dais is a type of platform)
'welshman's building' is the wordplay.
'welshman' becomes 'Dai' (Welsh name).
'building' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'dai'+'s'='DAIS'
Can you help me to learn more?
(Other definitions for dais that I've seen before include "stepping up here?" , "Raised floor" , "Platform; podium" , "Welshman's" , "support for speaker" .)