What does the cryptic crossword clue "New form for trainees missing item in the middle: it will need filling in (6)" mean?

The definition is "it will need filling in". The wordplay uses an anagram. anagram of RAINES = SARNIE The answer is SARNIE.

New form for trainees missing item in the middle: it will need filling in (6)

Guardian Cryptic #29096 | 19 Down | 2023-06-14 flower