Once again locks up fellow among left-wingers (7)
I believe the answer is:
remands
'once again locks up' is the definition.
'fellow among left-wingers' is the wordplay.
'fellow' becomes 'man' (I've seen this before).
'among' means one lot of letters goes inside another.
'left-wingers' becomes 'reds' (red is a kind of left-winger).
'man' placed into 'reds' is 'REMANDS'.
(Other definitions for remands that I've seen before include "Returns to custody pending further evidence" , "Returns prisoner to custody while further evidence is sought" , "Returns prisoner to custody while investigation proceeds" , "Sends back" , "Consigns to prison to await trial" .)