It's just like the male to be the aggressor (5)
I believe the answer is:
bully
'the aggressor' is the definition.
(bully is a kind of aggressor)
'it's just like the male' is the wordplay.
I cannot really see how this works, but
'like' could be 'ly' (similar in meaning) and 'ly' is found in the answer.
This explanation may well be incorrect...
'to be' is the link.
Can you help me to learn more?
(Other definitions for bully that I've seen before include "excellent" , "Well done" , "Very good" , "Playground thug" , "Intimidator" .)