Movement follows start of fat camp (7)
I believe the answer is:
faction
'camp' is the definition.
(I know that faction is a type of camp)
'movement follows start of fat' is the wordplay.
'movement' becomes 'action' (I've seen this in another clue).
'follows' says to put letters next to each other.
'start 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 "Seeing political dissenters" , "Clique that seeks power often through intrigue" , "Small trouble-making group within a group" , "Dissenting group - drama documentary" , "Act on if for a small group within a larger one" .)