This compiler's occasionally obtuser fill (5)
I believe the answer is:
imbue
'fill' is the definition.
(imbue means to fill)
'this compiler's occasionally obtuser' is the wordplay.
'this compiler's' becomes 'im' ('I am' referring to the compiler of the crossword).
'occasionally' indicates alternate letters.
The alternate letters of 'obtuser' are 'bue'.
'im'+'bue'='IMBUE'
(Other definitions for imbue that I've seen before include "dye" , "Instil - soak" , "Permeate - inspire" , "Suffuse with colour" , "Spread or diffuse through; inspire" .)