Feasible that sixth beer contains boron (6)
I believe the answer is:
viable
'feasible' is the definition.
(I know that feasible can be written as viable)
'sixth beer contains boron' is the wordplay.
'sixth' becomes 'VI' (Roman numerals).
'beer' becomes 'ale' (type of beer).
'contains' means one lot of letters goes inside another.
'boron' becomes 'B' (synonyms).
'ale' placed around 'b' is 'able'.
'vi'+'able'='VIABLE'
'that' acts as a link.
(Other definitions for viable that I've seen before include "Workable" , "Fumbled" , "(Of plan) capable of working" , "Able to survive; practicable" , "having potential to succeed" .)