Teen gamer excited by contract (9)
I believe the answer is:
agreement
'contract' is the definition.
(a contract is a legal agreement)
'teen gamer excited' is the wordplay.
'excited' indicates anagramming the letters.
'teen'+'gamer'='teengamer'
'teengamer' with letters rearranged gives 'AGREEMENT'.
'by' is the link.
(Other definitions for agreement that I've seen before include "consensus" , "legal contract maybe" , "deal" , "match" , "Unlikely result" .)