Take a firm line with trendy little girl — end of argument (6)
I believe the answer is:
insist
'take a firm line with' is the definition.
I can't tell whether this definition defines the answer.
'trendy little girl end of argument' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'little girl' becomes 'sis' (short for sister).
'end of' suggests the final letters.
The final letter of 'argument' is 't'.
'in'+'sis'+'t'='INSIST'
Can you help me to learn more?
(Other definitions for insist that I've seen before include "Refuse to give way" , "Persist in pressing" , "Beg persistently and urgently" , "Take a firm stand" , "Be firmly demanding" .)