Mistake in opening of ten excellent houses (7)
I believe the answer is:
terrace
'mistake in opening of ten excellent houses' is the definition.
'terrace' can be an answer for 'houses' (I've seen this before). I am unsure of the 'mistake in opening of ten excellent' bit.
'mistake in opening of ten excellent houses' is the wordplay.
'mistake in' becomes 'err' ('err' can be a synonym of 'mistake'. I am not sure about the 'in' bit.**).
'opening of' indicates taking the first letters.
'excellent' becomes 'ace' (I've seen this before).
'houses' is an insertion indicator.
The first letter of 'ten' is 't'.
't'+'ace'='tace'
'err' inserted into 'tace' is 'TERRACE'.
Can you help me to learn more?
(Other definitions for terrace that I've seen before include "Stadium's standing area" , "Continuous row of houses" , "Row of attached houses" , "Urban housing" , "stand" .)