Occasionally Boursin and drink with partner initially (10)
I believe the answer is:
originally
'initially' is the definition.
(thesaurus)
'occasionally boursin and drink with partner' is the wordplay.
'occasionally' indicates alternate letters.
'and' says to put letters next to each other.
'drink' becomes 'gin' (gin is a kind of drink).
'with' says to put letters next to each other.
'partner' becomes 'ally' (I've seen this in other clues).
The alternating letters of 'boursin' are 'ori'.
'ori'+'gin'+'ally'='ORIGINALLY'
(Other definitions for originally that I've seen before include "In the first place" , "At the start" , "new way" , "to begin with" .)