Glib one becomes despicable (7)
I believe the answer is:
ignoble
'despicable' is the definition.
(I've seen this in another clue)
'glib one becomes' is the wordplay.
'becomes' indicates an anagram.
'glib'+'one'='glibone'
'glibone' is an anagram of 'IGNOBLE'.
(Other definitions for ignoble that I've seen before include "Not honourable, low" , "Completely lacking in character or purpose" , "Not worthy in character or purpose" , "No bilge! (anag)" , "not in the top class" .)