Nobleman missing river's underground movement (6)
I believe the answer is:
maquis
'underground movement' is the definition.
(French resistance group in WW2)
'nobleman missing river's' is the wordplay.
'nobleman' becomes 'marquis' (type of nobleman).
'missing' suggests deleting specific letters.
'river' becomes 'r' (abbreviation for river).
'marquis' with 'r' taken away is 'MAQUIS'.
(Other definitions for maquis that I've seen before include "underground movement" , "World War Two French resistance" , "French underground force" , "Frenchman fighting" , "dense scrubby vegetation around the Med" .)