Spoil function, getting formal (6)
I believe the answer is:
cosset
'spoil' is the definition.
(I know that spoil can be written as cosset)
'function getting formal' is the wordplay.
'function' becomes 'cos' (mathematical function).
'getting' means one lot of letters go next to another.
'formal' becomes 'set' (set can mean formal or conventional).
'cos'+'set'='COSSET'
(Other definitions for cosset that I've seen before include "Feather-bed" , "Fondle" , "Nanny" , "Pamper, coddle (6)" , "Care for overindulgently" .)