New form for trainees missing item in the middle: it will need filling in (6)
I believe the answer is:
sarnie
'new form for trainees missing item in the middle' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'it will need filling in' is the wordplay.
I cannot really see how this works, but
'it' could be 'sa' (SA can mean sex appeal, as in 'it') and 'sa' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sarnie that I've seen before include "Popular bread-based snack (colloq)" , "Bread and filling snack (informal)" , "Sandwich (slang)" , "no classy fare" , "Sandwich (informal)" .)