River guide to straighten out fool's garment (7)
I believe the answer is:
coxcomb
'fool's garment' is the definition.
(hat worn by a jester)
'river guide to straighten out' is the wordplay.
'river guide' becomes 'cox' (someone who directs a rowing crew).
'to straighten out' becomes 'comb' (combing is a kind of straightening out).
'cox'+'comb'='COXCOMB'
(Other definitions for coxcomb that I've seen before include "Fool" , "Fop - fleshy outgrowth on a cockerel's head" , "rather vain fellow" , "Red crest on the head of the domestic fowl" , "foppish man" .)