Elder son, he changes short trousers (10)
I believe the answer is:
lederhosen
'short trousers' is the definition.
(I've seen this before)
'elder son he changes' is the wordplay.
'changes' indicates anagramming the letters.
'elder'+'son'+'he'='eldersonhe'
'eldersonhe' is an anagram of 'LEDERHOSEN'.
(Other definitions for lederhosen that I've seen before include "trousers displayed" , "Clothing" , "Short leather trousers with braces" , "Bavarian shorts with braces" , "Alpine shorts" .)