If my rental was only 11 months of the year, can I claim partial mortgage interest for the one month ?

[TomK2023]

Assuming I understand you correctly, you rented out your property for 11 months, correct? 

If true, then you can prorate your mortgage interest and deduct that amount as a rental expense.  

For example, if the mortgage interest on your rental property is $1,200 for the whole year, then you can deduct $1,100 as a rental expense ($1,200 / 12 months = $100 per month x 11 months rented = $1,100).   See image below.

If you only rented it for one month, then your mortgage  interest rental expense is $100.