Notice mould being reversed — it's unmissable (4,3)
I believe the answer is:
must see
'unmissable' is the definition.
(I've seen this before)
'notice mould being reversed' is the wordplay.
I cannot really understand how this works, but
'notice' could be 'see' (notice can mean to spot or see) and 'see' is located in the answer.
'mould' could be 'must' (I've seen this before) and 'must' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for must see that I've seen before include "Wonderful picture" , "Set viewing" , "unmissable" .)