Pioneer’s guard dog crossing river (11)
I believe the answer is:
trendsetter
'pioneer's' is the definition.
(a trendsetter pioneers something new)
'guard dog crossing river' is the wordplay.
'guard' becomes 'tend' (I've seen this in another clue).
'dog' becomes 'setter' (setter is a type of dog).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'tend'+'setter'='tendsetter'
'tendsetter' going around 'r' is 'TRENDSETTER'.
(Other definitions for trendsetter that I've seen before include "Fashionista" , "Fashion leader" , "hip innovator?" .)