Moody learner into something sticky (4)
I believe the answer is:
glum
'moody' is the definition.
('glum' can be a synonym of 'moody')
'learner into something sticky' is the wordplay.
'learner' becomes 'l'.
'into' is an insertion indicator.
'something sticky' becomes 'gum'.
'l' put inside 'gum' is 'GLUM'.
(Other definitions for glum that I've seen before include "Dispirited, moody" , "Down in the mouth, depressed" , "Sad and gloomy" , "Melancholy" , "Sad and downcast" .)