Posers start leading in fashions (6)
I believe the answer is:
models
'posers' is the definition.
('model' can be a synonym of 'poser')
'start leading in fashions' is the wordplay.
'start leading' becomes 'l' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'fashions' becomes 'modes' ('mode' can be a synonym of 'fashion').
'l' inserted within 'modes' is 'MODELS'.
Can you help me to learn more?
(Other definitions for models that I've seen before include "Poses for artists" , "Human clothes horses or protoypes" , "Exemplars or mannequins" , "Prototypes" , "Representative forms or patterns" .)