Movement follows opening of fat camp (7)
I believe the answer is:
faction
'camp' is the definition.
(I've seen this before)
'movement follows opening of fat' is the wordplay.
'movement' becomes 'action'.
'follows' is a charade indicator (letters next to each other).
'opening of' says to take the initial letters.
The first letter of 'fat' is 'f'.
'action' put after 'f' is 'FACTION'.
(Other definitions for faction that I've seen before include "Rebellious group" , "A dissenting clique" , "Small united group within larger one" , "Small trouble-making group within a group" , "part of party" .)