Fall for one wearing incredible short dress coat (11)
I believe the answer is:
swallowtail
'dress coat' is the definition.
'fall for one wearing incredible short' is the wordplay.
'fall for' becomes 'swallow'.
'one' becomes 'i' (Roman numeral).
'wearing' means one lot of letters goes inside another.
'incredible' becomes 'tall' (as in a tall tale).
'short' means to remove the last letter.
'tall' with its final letter removed is 'tal'.
'i' placed inside 'tal' is 'tail'.
'swallow'+'tail'='SWALLOWTAIL'
(Other definitions for swallowtail that I've seen before include "In circle, mythical snake does" , "insect" , "coat" , "Butterfly" .)