Agitated cat stuck in tree with last bit of stale biscuit (7)
I believe the answer is:
oatcake
'biscuit' is the definition.
(I know that oatcake is a type of cracker)
'agitated cat stuck in tree with last bit of stale' is the wordplay.
'agitated' indicates anagramming the letters.
'stuck in' means one lot of letters goes inside another.
'tree' becomes 'oak' (oak is a type of tree).
'with' means one lot of letters go next to another.
'last bit of' says to take the final letters.
The final letter of 'stale' is 'e'.
'cat' with letters rearranged gives 'tca'.
'tca' inserted into 'oak' is 'oatcak'.
'oatcak'+'e'='OATCAKE'
(Other definitions for oatcake that I've seen before include "Thin biscuit made of ground grain" , "something like a biscuit" , "Savoury biscuit" , "Traditional Scottish biscuit" , "Dry biscuit" .)