Lorry starts to alarm rider with uncontrolled movement (5)
I believe the answer is:
artic
'lorry' is the definition.
(informal term for an articulated lorry)
'starts to alarm rider with uncontrolled movement' is the wordplay.
'starts to' suggests taking the first letters.
'with' says to put letters next to each other.
'uncontrolled movement' becomes 'tic' (involuntary bodily movement).
The initial letters of 'alarm rider' is 'ar'.
'ar'+'tic'='ARTIC'
(Other definitions for artic that I've seen before include "Manoeuvrable lorry" , "Region around the North Pole" , "Lorry (abbrev.)" , "Large vehicle" , "This transports freight" .)