Mild Liberal entering group (5)
I believe the answer is:
bland
'mild' is the definition.
(thesaurus)
'liberal entering group' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'entering' means one lot of letters goes inside another.
'group' becomes 'band' (band is a kind of group**).
'l' placed within 'band' is 'BLAND'.
(Other definitions for bland that I've seen before include "without distinctive characteristics" , "Without flavour" , "Suave" , "Mild and insipid" , "Unruffled but unstimulating" .)