Starts to round up reindeer at large in the countryside (5)
I believe the answer is:
rural
'in the countryside' is the definition.
(I've seen this before)
'starts to round up reindeer at large' is the wordplay.
'starts to' suggests taking the first letters.
'large' becomes 'l'.
The first letters of 'round up reindeer at' is 'rura'.
'rura'+'l'='RURAL'
(Other definitions for rural that I've seen before include "living in the countryside" , "Bucolic; pastoral" , "Agrestic" , "Countryfied" , "Connected with farming" .)