First signs of horsefly's unwelcome, exasperating appearance (3)
I believe the answer is:
hue
'appearance' is the definition.
(I've seen this before)
'first signs of horsefly's unwelcome exasperating' is the wordplay.
'first signs of' suggests taking the first letters.
The first letters of 'horseflys unwelcome exasperating' is 'HUE'.
(Other definitions for hue that I've seen before include "Shade of colour; Vietnam port" , "appearance" , "General outburst of alarm or protest - . . . and cry" , "Tint, shade" , "Tinge" .)