Cat flap within reach (5)
I believe the answer is:
tabby
'cat' is the definition.
(tabby is a kind of cat)
'flap within reach' is the wordplay.
'flap' becomes 'tab' (I've seen this before).
'within reach' becomes 'by' (I am not sure about this - if you are sure you should believe this answer much more).
'tab'+'by'='TABBY'
Can you help me to learn more?
(Other definitions for tabby that I've seen before include "Mottled and striped grey or brownish cat" , "Domestic cat" , "Brownish cat with dark streaks" , "Brownish-grey cat" , "Cat with a mottled coat" .)