Welshman who in Paris drinks Irish cocktail (8)
I believe the answer is:
daiquiri
'cocktail' is the definition.
(I know that Daiquiri is a type of cocktail)
'welshman who in paris drinks irish' is the wordplay.
'welshman' becomes 'dai' (Welsh name).
'who in paris' becomes 'qui' (French for 'who').
'drinks' means one lot of letters goes inside another.
'irish' becomes 'IR' (abbreviation).
'qui' enclosing 'ir' is 'quiri'.
'dai'+'quiri'='DAIQUIRI'
(Other definitions for daiquiri that I've seen before include "Cocktail made with rum and lime or lemon juice" , "A cocktail of rum, lime juice etc" , "Iced drink of rum, lime juice and sugar" , "Rum-based cocktail" , "Type of cocktail" .)