Stick on after bumping head (6)
I believe the answer is:
adhere
'stick' is the definition.
('adhere' can be a synonym of 'stick')
'on after bumping head' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to'**).
'after' means one lot of letters go next to another.
'bumping' is an anagram indicator.
'head' with letters rearranged gives 'adhe'.
're' put after 'adhe' is 'ADHERE'.
(Other definitions for adhere that I've seen before include "Bond" , "Stick like a stamp" , "Stick to or continue faithful" , "Stick or cleave to" , "Header (anag)" .)