Ancient English gum (5)
I believe the answer is:
paste
'gum' is the definition.
(I've seen this before)
'ancient english' is the wordplay.
'ancient' becomes 'past' ('past' can be similar in meaning to 'ancient').
'english' becomes 'e' (abbreviation).
'past'+'e'='PASTE'
(Other definitions for paste that I've seen before include "food preparation" , "Watery adhesive" , "Liquid glue" , "(In computing) insert text into a document" , "Sticky stuff" .)