To understand, the female must be mentally composed (8)
I believe the answer is:
together
'mentally composed' is the definition.
(together can mean stable or collected)
'to understand the female' is the wordplay.
'understand' becomes 'get' (getting is a kind of understanding).
'the female' becomes 'her' (I've seen this before).
'to'+'get'+'her'='TOGETHER'
'must be' acts as a link.
(Other definitions for together that I've seen before include "Concertedly" , "One with another, simultaneously" , "in league" , "Married" , "Level-headed" .)