Cat flap alongside (5)
I believe the answer is:
tabby
'cat' is the definition.
(I know that tabby is a type of cat)
'flap alongside' is the wordplay.
'flap' becomes 'tab' (both can mean a small piece of material).
'alongside' becomes 'by' (I've seen this in other clues).
'tab'+'by'='TABBY'
(Other definitions for tabby that I've seen before include "Brownish cat with dark streaks" , "Cat with striped markings" , "Cat with grey or brown streaks or patches" , "Streaked cat" , "Mottled and striped grey or brownish cat" .)