Ridiculous trousers start to expand, and pop undone (12)
I believe the answer is:
preposterous
'ridiculous' is the definition.
('preposterous' can be a synonym of 'ridiculous')
'trousers start to expand and pop undone' is the wordplay.
'start to' suggests taking the first letters.
'and' means one lot of letters go next to another (I've seen this in other clues).
'undone' indicates an anagram.
The initial letter of 'expand' is 'e'.
'trousers'+'e'='trouserse'
'trouserse' after 'pop' is 'poptrouserse'.
'poptrouserse' anagrammed gives 'PREPOSTEROUS'.
(Other definitions for preposterous that I've seen before include "Unreasonable - fatuous" , "Quite absurd" , "Ludicrous" , "Laughable" , "Utterly absurd" .)