Gasp at French eccentric's baggy trousers (10)
I believe the answer is:
pantaloons
'baggy trousers' is the definition.
(I've seen this before)
'gasp at french eccentric's' is the wordplay.
'gasp' becomes 'pant' (synonyms).
'at french' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'eccentric's' becomes 'loon's'.
'pant'+'a'+'loons'='PANTALOONS'
Can you help me to learn more?
(Other definitions for pantaloons that I've seen before include "Women's baggy trousers" , "Baggy trousers gathered at the ankles" .)