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 before)
'back with a dizzy head' is the wordplay.
'back' becomes 'aft' (associated in meaning).
'with' means one lot of letters go next to another.
'head' says to take the initial letters.
The first letter of 'dizzy' is 'd'.
'aft' after 'd' is 'DAFT'.
'to get' is the 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" .)