Significance adult males observed in witty saying (6)
I believe the answer is:
moment
'significance' is the definition.
(moment is a kind of significance)
'adult males observed in witty saying' is the wordplay.
'adult males' becomes 'men' ('man' can be a synonym of 'adult male').
'observed in' indicates putting letters inside.
'witty saying' becomes 'mot' (I've seen this before).
'men' put into 'mot' is 'MOMENT'.
(Other definitions for moment that I've seen before include "tick" , "little while" , "Indefinitely short time" , "A very brief portion of time" , "Second, importance" .)