Stir-crazy, whizz about to start with (7)
I believe the answer is:
firstly
'to start with' is the definition.
(I've seen this in another clue)
'stir-crazy whizz about' is the wordplay.
'crazy' is an anagram indicator.
'whizz' becomes 'fly' (I am not sure about this - if you are sure you should believe this answer much more).
'about' means one lot of letters goes inside another.
'stir' with letters rearranged gives 'irst'.
'irst' placed inside 'fly' is 'FIRSTLY'.
Can you help me to learn more?
(Another definition for firstly that I've seen is " Primarily".)