Gassy very old beer's off (7)
I believe the answer is:
verbose
'gassy' is the definition.
(thesaurus)
'very old beer's off' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'off' is an anagram indicator.
'o'+'beers'='obeers'
'obeers' anagrammed gives 'erbose'.
'v'+'erbose'='VERBOSE'
(Other definitions for verbose that I've seen before include "Using more words than are needed" , "Observe (anag) -- long-winded" , "With an excess of words" , "Using or containing too many words" , "not given to Trappist ways" .)