Looks a bit like glue, but this isn't a sticky one (4)
I believe the answer is:
clue
'looks a bit like glue but this isn't a sticky one' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for clue that I've seen before include "One of twenty-four in this puzzle" , "contribution to crossword" , "Hint to get what this is?" , "crossword guide" , "Slight indication (as here)" .)