Competition solicitor adopting unbalanced manner (10)
I believe the answer is:
tournament
'competition' is the definition.
(tournament is a kind of competition)
'solicitor adopting unbalanced manner' is the wordplay.
'solicitor' becomes 'tout' (someone who solicits business).
'adopting' is an insertion indicator (inserted letters are taken on or adopted).
'unbalanced' indicates anagramming the letters.
'manner' anagrammed gives 'rnamen'.
'tout' enclosing 'rnamen' is 'TOURNAMENT'.
(Other definitions for tournament that I've seen before include "open perhaps" , "pageant" , "meeting!" , "Jousting competition" , "military display" .)