Getting older, awful gag in English (6)
I believe the answer is:
ageing
'getting older' is the definition.
('age' can be a synonym of 'get older')
'awful gag in english' is the wordplay.
'awful' indicates an anagram.
'english' becomes 'e' (abbreviation).
'gag'+'in'+'e'='gagine'
'gagine' is an anagram of 'AGEING'.
(Other definitions for ageing that I've seen before include "Growing old" , "no longer fresh" , "getting more advanced?" , "Elderly" , "Declining" .)