Four Across done first of all? Fancy! (3)
I believe the answer is:
fad
'fancy' is the definition.
(I've seen this in other clues)
'four across done first of all?' is the wordplay.
'first of all?' suggests taking the first letters (I've seen 'first of all' mean this).
The initial letters of 'four across done' is 'FAD'.
(Other definitions for fad that I've seen before include "Personal whim" , "A craze or current vogue" , "Passionate interest" , "Short-lived craze" , "Temporary vogue or craze" .)