I am upset about fashionable skirt (4)
I believe the answer is:
mini
'skirt' is the definition.
(a mini skirt)
'i am upset about fashionable' is the wordplay.
'i am' becomes 'I'm' (contraction).
'upset' is a reversal indicator.
'about' is a reversal indicator.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'im' backwards is 'mi'.
'in' in reverse letter order is 'ni'.
'mi'+'ni'='MINI'
(Other definitions for mini that I've seen before include "Once fashionable skirt" , "type of garment" , "Little - car" , "Italian Job car?" , "Iconic economy car (introduced in 1959)" .)