No longer trendy: lot had changed (3,3)
I believe the answer is:
old hat
'no longer trendy' is the definition.
I can't tell whether this definition defines the answer.
'lot had changed' is the wordplay.
'changed' is an anagram indicator.
'lot'+'had'='lothad'
'lothad' anagrammed gives 'OLD-HAT'.
Can you help me to learn more?
(Other definitions for old hat that I've seen before include "Out-of-date" , "Something out of date" , "Stale - out of date" , "Out of fashion, like ancient headgear" , "On the way out" .)