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Bird following two big signs hit the roof (4,2)

Ross

I believe the answer is:

lost it

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

'signs hit the roof' is the definition.
I can't tell whether this defines the answer.

'bird following two big' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'following' is a charade indicator (letters next to each other).
'two big' becomes 'los' (I can't explain this - if you can you should believe this answer much more).
'tit' put after 'los' is 'LOST IT'.

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