Yet this is how all-seater stadiums are (15)
I believe the answer is:
notwithstanding
'yet' is the definition.
(thesaurus)
'this is how all-seater stadiums are' is the wordplay.
I cannot quite see how this works, but
an anagram of 'this' is 'iths' which is present in the answer.
'are' could be 'a' (the 'are' is a metric unit of area) and 'a' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for notwithstanding that I've seen before include "... in spite" , "without being still" , "Nevertheless" , "Even so" , "However" .)