Male idiot behind opening of Avengers Assemble (5)
I believe the answer is:
amass
'assemble' is the definition.
(I've seen this before)
'male idiot behind opening of avengers' is the wordplay.
'male' becomes 'm' (common abbreviation).
'idiot' becomes 'ass' (I've seen this before).
'behind' is a charade indicator (letters next to each other).
'opening of' indicates taking the first letters.
The first letter of 'avengers' is 'a'.
'm'+'ass'='mass'
'mass' put after 'a' is 'AMASS'.
(Other definitions for amass that I've seen before include "Gather together, pile up" , "Put together" , "Accrue, heap up" , "Stock up on" , "Sodium chloride" .)