Welshman in Paris, who keeps one recipe for a cocktail (8)
I believe the answer is:
daiquiri
'a cocktail' is the definition.
(daiquiri is a kind of cocktail)
'welshman in paris who keeps one recipe' is the wordplay.
'welshman' becomes 'dai' (I've seen this before).
'in paris who' becomes 'qui' (French for 'who').
'keeps' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'recipe' becomes 'r' (abbreviation).
'i'+'r'='ir'
'qui' enclosing 'ir' is 'quiri'.
'dai'+'quiri'='DAIQUIRI'
'for' is the link.
(Other definitions for daiquiri that I've seen before include "Rum-based cocktail" , "Cocktail made with rum and lime or lemon juice" , "Iced drink of rum, lime juice and sugar" , "A cocktail of rum, lime juice etc" , "Type of cocktail" .)