Second sign delivered in extremes of merriment (6)
I believe the answer is:
moment
'second' is the definition.
(synonyms)
'sign delivered in extremes of merriment' is the wordplay.
'sign' becomes 'omen' (omen is a kind of sign).
'delivered in' indicates putting letters inside.
'extremes of' suggests removing the centre.
'merriment' with its centre removed is 'mt'.
'omen' placed within 'mt' is 'MOMENT'.
(Other definitions for moment that I've seen before include "Short time - importance" , "force to turn things around" , "tick" , "Instant - importance" , "Import" .)