Rest that makes one smile, as it can occasionally (6)
I believe the answer is:
siesta
'rest' is the definition.
(I've seen this before)
'one smile as it can occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
'smile'+'as'+'it'+'can'='smileasitcan'
The alternate letters of 'smileasitcan' are 'SIESTA'.
'that makes' is the link.
(Other definitions for siesta that I've seen before include "After lunch nap" , "Afternoon relaxation" , "Daytime nap" , "Afternoon nap in Cadiz" , "Diurnal nap in Spain" .)