Mediocre line in Lear to be rewritten (9)
I believe the answer is:
tolerable
'mediocre' is the definition.
(thesaurus)
'line in lear to be rewritten' is the wordplay.
'line' becomes 'l'.
'in' is an insertion indicator.
'rewritten' is an anagram indicator.
'lear'+'to'+'be'='leartobe'
'leartobe' is an anagram of 'toerable'.
'l' placed inside 'toerable' is 'TOLERABLE'.
(Other definitions for tolerable that I've seen before include "Adequate" , "Acceptable" , "Passable" , "may be accepted?" , "Fair" .)