Smooth learner enters the ring (5)
I believe the answer is:
bland
'smooth' is the definition.
(thesaurus)
'learner enters the ring' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'enters' means one lot of letters goes inside another.
'the ring' becomes 'band' ('band' can be a synonym of 'ring').
'l' put inside 'band' is 'BLAND'.
(Other definitions for bland that I've seen before include "Insipid; lacklustre" , "Lacking taste, flavour" , "Flavourless" , "Mild; dull" , "Insipid, vapid" .)