Biscuit: a form of tack consumed by Old English (7)
I believe the answer is:
oatcake
'biscuit' is the definition.
(I've seen this before)
'a form of tack consumed by old english' is the wordplay.
'form of' indicates anagramming the letters (the letters need to be in another 'form').
'consumed by' is an insertion indicator.
'old english' becomes 'oe'.
'tack' with letters rearranged gives 'tcak'.
'a'+'tcak'='atcak'
'atcak' placed inside 'oe' is 'OATCAKE'.
(Other definitions for oatcake that I've seen before include "Dry biscuit" , "Thin biscuit made of ground grain" , "Savoury biscuit" , "Traditional Scots biscuit" , "Scots biscuit-like food" .)