Gussalufz’s dull spear (6)
I believe the answer is:
impale
'spear' is the definition.
(I've seen this in another clue)
'gussalufz's dull' is the wordplay.
'gussalufz's' becomes 'I'm' (where the crossword setter is 'Gussalufz').
'dull' becomes 'pale' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'im'+'pale'='IMPALE'
Can you help me to learn more?
(Other definitions for impale that I've seen before include "go through with it" , "Fix as on a sharp stick" , "Transfix with spear" , "transfix with sharp item" , "shut in as before" .)