Adult breaking tenner is loaded? Seriously? (2,7)
I believe the answer is:
in earnest
'seriously?' is the definition.
(synonyms)
'adult breaking tenner is loaded?' is the wordplay.
'adult' becomes 'a' (abbreviation).
'breaking' is an insertion indicator.
'loaded?' indicates an anagram (loaded can mean drunk).
'tenner'+'is'='tenneris'
'tenneris' is an anagram of 'inernest'.
'a' inserted into 'inernest' is 'IN EARNEST'.
(Other definitions for in earnest that I've seen before include "seriously" , "With serious intentions" , "With serious intention, not fooling about (2.7)" , "with enthusiasm" , "Sincere in intention" .)