Lacking wide interests, ruins a learner sadly (7)
I believe the answer is:
insular
'lacking wide interests' is the definition.
(insular can mean narrow-minded)
'ruins a learner sadly' is the wordplay.
'learner' becomes 'l'.
'sadly' indicates an anagram.
'ruins'+'a'+'l'='ruinsal'
'ruinsal' is an anagram of 'INSULAR'.
(Other definitions for insular that I've seen before include "Surrounded by water - narrow - prejudiced" , "Aloof - like an island" , "Narrow-minded like islander?" , "Narrow-minded; isolated" , "cut off" .)