Screen say needs moving: it's vital (9)

I believe the answer is:

necessary

I'm a little stuck... Click here to teach me more about this clue!

'vital' is the definition.
(I've seen this before)

'screen say needs moving' is the wordplay.
'screen say' anagrams to 'NECESSARY'.
But, I'm not sure how this is indicated.

'it's' is the link.

Can you help me to learn more?

(Other definitions for necessary that I've seen before include "obligatory" , "Vital" , "Required, needed" , "Requisite" , "Absolutely essential" .)

I've seen this clue in The Sun.

Want a hint initially instead of a full solution? Install my app