Disturb informed group (5)
I believe the answer is:
upset
'disturb' is the definition.
(I know that disturb can be written as upset)
'informed group' is the wordplay.
'informed' becomes 'up' (being up on a topic means being informed about it).
'group' becomes 'set' (both can mean a collection).
'up'+'set'='UPSET'
(Other definitions for upset that I've seen before include "Overturn - worry" , "Overturn or unsettle" , "Overturn or discompose" , "Overturn, tip over" , "Indisposition" .)