Hold forth but sound lame after last month (7)
I believe the answer is:
declaim
'hold forth' is the definition.
(I've seen this before)
'sound lame after last month' is the wordplay.
I cannot really understand how this works, but
'month' could be 'dec' (abbreviation for December) and 'dec' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'but' acts as a link.
Can you help me to learn more?
(Other definitions for declaim that I've seen before include "Harangue" , "Rail against" , "attack" , "Deliver an impassioned speech" , "Hold forth" .)