Key near cat (5)
I believe the answer is:
tabby
'cat' is the definition.
(I know that tabby is a type of cat)
'key near' is the wordplay.
'key' becomes 'tab' (key on a computer keyboard).
'near' becomes 'by' (near means close by).
'tab'+'by'='TABBY'
(Other definitions for tabby that I've seen before include "Cat with grey or brown streaks or patches" , "Domestic cat" , "Streaked cat" , "Grey-brown cat" , "Brownish-grey cat" .)