A holiday in February - except in a leap year (6)

'a holiday in february except in a leap year' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'a holiday in february except in a leap year' is the wordplay.
I cannot quite understand how this works, but
'a' is found in the answer.
'year' could be 'y' and 'y' is found in the answer.
The remaining letters 'doff' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?