Man on tube line turning indecent (13)
I believe the answer is:
unmentionable
'indecent' is the definition.
(thesaurus)
'man on tube line turning' is the wordplay.
'turning' indicates anagramming the letters.
'man'+'on'+'tube'+'line'='manontubeline'
'manontubeline' is an anagram of 'UNMENTIONABLE'.
(Other definitions for unmentionable that I've seen before include "Taboo" , "Forbidden as a topic of conversation" , "Not fit to be referred to" .)