Member of a popular movement (3)
I believe the answer is:
arm
'member' is the definition.
(I've seen this before)
'of a popular movement' is the wordplay.
'of' indicates a hidden word.
'ARM' is hidden in the letters of 'popular movement'.
(Other definitions for arm that I've seen before include "Sleeve" , "Test of strength with elbows on the table" , "Activate fuse" , "What soldiers do" , "Supply weapons" .)