Adult males have the last word (4)
I believe the answer is:
amen
'the last word' is the definition.
(I've seen this before)
'adult males' is the wordplay.
'adult' becomes 'a'.
'males' becomes 'men' (man is a kind of male).
'a'+'men'='AMEN'
'have' acts as a link.
(Other definitions for amen that I've seen before include "Churchgoer's assent?" , "Last word of prayer" , "I agree!" , "Prayerful last word" , "Word at the end of a prayer" .)