Are beside the notice urging you to enlist (6)
I believe the answer is:
adjoin
'enlist' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'are beside the notice urging you' is the wordplay.
'are beside' is a charade indicator (letters next to each other).
'the notice' becomes 'ad' (both can mean an advertisement).
'urging you' becomes 'join' (I am not sure about this - if you are sure you should believe this answer much more).
'ad' put next to 'join' is 'ADJOIN'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for adjoin that I've seen before include "Meet - touch" , "20 down" , "Abut, border" , "Be next or nearest to" , "Neighbour" .)