Extremes of gastrointestinal irritation? Hiccup! (6)
I believe the answer is:
glitch
'hiccup' is the definition.
(I've seen this before)
'extremes of gastrointestinal irritation?' is the wordplay.
'extremes of' means to remove the middle letters.
'irritation?' becomes 'itch' (I've seen this before).
'gastrointestinal' with its middle removed is 'gl'.
'gl'+'itch'='GLITCH'
(Other definitions for glitch that I've seen before include "Computer problem" , "An informal malfunction" , "Unexpected failure or error" , "Irregular computer failure" , "Find fault" .)