Stick the head is upset about! (6)
I believe the answer is:
adhere
'stick' is the definition.
(to adhere to something is to stick to it)
'head is upset about' is the wordplay.
'is upset' indicates anagramming the letters (I've seen 'are upset' mean this).
'about' becomes 're' (regarding).
'head' with letters rearranged gives 'adhe'.
'adhe'+'re'='ADHERE'
'the' is the link.
(Other definitions for adhere that I've seen before include "Stay attached" , "Stick - join" , "'Stick to, a play say (6)'" , "Stick or cleave to" , "Remain attached" .)