It's silly to get back with a dizzy head (4)
I believe the answer is:
daft
'it's silly' is the definition.
(I've seen this in other clues)
'back with a dizzy head' is the wordplay.
'back' becomes 'aft' (associated in meaning).
'with' says to put letters next to each other.
'head' indicates taking the first letters.
The initial letter of 'dizzy' is 'd'.
'aft' after 'd' is 'DAFT'.
'to get' acts as a link.
(Other definitions for daft that I've seen before include "Informally silly or foolish" , "'Silly, foolish (4)'" , "Bats!" , "'---- as a brush': dotty" , "Informally silly or crazy" .)