Morning before adult sons gather (5)
I believe the answer is:
amass
'gather' is the definition.
(amass can mean to collect or gather)
'morning before adult sons' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'before' says to put letters next to each other.
'adult' becomes 'a' (abbreviation).
'sons' becomes 'ss' (genealogical abbreviation for son written twice).
'am'+'a'+'ss'='AMASS'
(Other definitions for amass that I've seen before include "store" , "Accumulate, heap up" , "Accumulate over time" , "Put together" , "Gathers" .)