On both sides of gate, notice a gnome (5)
I believe the answer is:
adage
'gnome' is the definition.
(I've seen this in another clue)
'on both sides of gate notice a' is the wordplay.
'on' says to put letters next to each other.
'both sides of' means to remove the middle letters (only the letters on the sides remain).
'notice' becomes 'ad' (short for advert).
'gate' with its middle taken out is 'ge'.
'ad'+'a'='ada'
'ge' put after 'ada' is 'ADAGE'.
(Other definitions for adage that I've seen before include "Slogan" , "Proverb, maxim" , "Maxim, aphorism" , "Well-remembered proverb or saying" , "True proverb or saying" .)