Learner, trouble-maker being slack (4)
I believe the answer is:
limp
'slack' is the definition.
(thesaurus)
'learner trouble-maker' is the wordplay.
'learner' becomes 'l'.
'trouble-maker' becomes 'imp'.
'l'+'imp'='LIMP'
'being' is the link.
(Other definitions for limp that I've seen before include "Weak" , "exhausted" , "Lacking stiffness" , "Hobble, being slack" , "Floppy" .)