He comes but once a year (4)
I believe the answer is:
noel
'a year' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'he comes but' is the wordplay.
'he' becomes 'Noe'.
'comes but' becomes 'l' (I am not sure about this - if you are sure you should believe this answer much more).
'noe'+'l'='NOEL'
'once' acts as a link.
Can you help me to learn more?
(Other definitions for noel that I've seen before include "Feature of last month" , "Christmas carol" , "Religious festival" , "Christmas, esp. in carols; boy's name" , "seasonal festival" .)