Invoice occasionally sent for accommodation (6)
I believe the answer is:
billet
'accommodation' is the definition.
(I've seen this before)
'invoice occasionally sent' is the wordplay.
'invoice' becomes 'bill' (synonyms).
'occasionally' indicates alternate letters.
The alternate letters of 'sent' are 'et'.
'bill'+'et'='BILLET'
'for' acts as a link.
(Other definitions for billet that I've seen before include "Housing for military personnel" , "Soldiers' quarters" , "living quarters" , "Piece of wood - bar of metal - place to stay" , "Lodgings for troops or evacuees" .)