Top fashion party in irrational fear (6)
I believe the answer is:
fedora
'top fashion' is the definition.
(I've seen this before)
'party in irrational fear' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'in' is an insertion indicator.
'irrational' indicates anagramming the letters.
'fear' with letters rearranged gives 'fera'.
'do' put inside 'fera' is 'FEDORA'.
(Other definitions for fedora that I've seen before include "Hat for RAF doe" , "Soft felt hat with a curled brim" , "Felt hat with a creased crown" , "Felt hat dented lengthways" , "top fashion" .)