Urge earnestly (6)
I believe the answer is:
exhort
'urge earnestly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for exhort that I've seen before include "Strongly encourage (someone to action)" , "Encourage, urge" , "Call" , "Give great encouragement to" , "Urge or advise strongly" .)