Be very sorry, but it’s weak theatre (6)

I believe the answer is:

lament

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'be very sorry but' is the definition.
I can't tell whether this definition defines the answer.

'weak theatre' is the wordplay.
'weak' becomes 'lame' ('lame' can be similar in meaning to 'weak').
'theatre' becomes 'nt' (National Theatre).
'lame'+'nt'='LAMENT'

'it's' acts as a link.

Can you help me to learn more?

(Other definitions for lament that I've seen before include "Express sorrow" , "complaint" , "Express disappointment" , "Mourn, grieve for" , "Mourn , bewail the loss of" .)

I've seen this clue in the Evening Standard.

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