Possibly agree with ten or one less than twenty (8)
I believe the answer is:
teenager
'one less than twenty' is the definition.
'possibly agree with ten' is the wordplay.
'possibly' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
'agree' put after 'ten' is 'tenagree'.
'tenagree' is an anagram of 'TEENAGER'.
'or' is the link.
(Other definitions for teenager that I've seen before include "Youngster" , "Old child or young adult?" , "14, perhaps" , "one with an unpredictable nature" , "one that's not yet made a score?" .)