Veg giving little brother dreadful colic (8)
I believe the answer is:
broccoli
'veg' is the definition.
(type of vegetable)
'giving little brother dreadful colic' is the wordplay.
'giving' means one lot of letters go next to another.
'little brother' becomes 'bro'.
'dreadful' indicates anagramming the letters.
'colic' is an anagram of 'ccoli'.
'bro'+'ccoli' is 'BROCCOLI'.
(Other definitions for broccoli that I've seen before include "Vegetable with tightly packed green or purple flower heads" , "Food" , "Type of vegetable" , "green veg" , "Variety of cabbage, green heads" .)