Cat wants to mew, firstly (3)
I believe the answer is:
tom
'cat' is the definition.
(a tom is a male cat)
'to mew firstly' is the wordplay.
'firstly' says to take the initial letters.
The first letter of 'mew' is 'm'.
'to'+'m'='TOM'
'wants' acts as a link.
(Other definitions for tom that I've seen before include "A male domestic cat" , "Male feline" , "Jerry's cartoon antagonist" , "Waits, perhaps" , "Boy - cat" .)