Unable to move at speed (4)
I believe the answer is:
fast
I believe this is a double definition.
'unable to move' is the first definition.
(I've seen this before)
'speed' is the second definition.
(I've seen this before)
'at' acts as a link.
(Other definitions for fast that I've seen before include "Don't scoff" , "not lento" , "Abstain from eating" , "Quick; firmly fixed" , "Accelerated" .)