Sticking head on is tricky (8)
I believe the answer is:
adhesion
'sticking' is the definition.
(I've seen this before)
'head on is tricky' is the wordplay.
'tricky' is an anagram indicator.
'head'+'on'+'is'='headonis'
'headonis' anagrammed gives 'ADHESION'.
(Other definitions for adhesion that I've seen before include "Adonis, he may be a sticker" , "Process of sticking together" , "Tackiness" , "Sticking fast" , "steady attachment" .)