Popular female soldier's child (6)
I believe the answer is:
infant
'child' is the definition.
(I know that infant is a type of child)
'popular female soldier's' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'female' becomes 'f' (common abbreviation).
'soldier' becomes 'ant' (soldier is a type of ant).
'in'+'f'+'ant'='INFANT'
(Other definitions for infant that I've seen before include "Tot" , "Wee one" , "Small boy" , "Small baby" , "Very young child or baby" .)