Not on high ground when there is less traffic (3-4)
I believe the answer is:
off-peak
'ground when there is less traffic' is the definition.
I can't tell whether this definition defines the answer.
'not on high' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'high' becomes 'peak' (similar in meaning).
'off'+'peak'='OFF-PEAK'
Can you help me to learn more?
(Other definitions for off-peak that I've seen before include "Time when tickets are usually cheaper" , "Outside the times when demand is greatest" , "Not at the time of highest demand" , "Fake fop (anag.)" , "Less busy (time)" .)