Cambridge blue, perhaps wore pants on river (5)
I believe the answer is:
rower
'cambridge blue perhaps' is the definition.
'wore pants on river' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'river' becomes 'r' (**).
'wore' anagrammed gives 'ower'.
'ower' enclosing 'r' is 'ROWER'.
(Other definitions for rower that I've seen before include "Stroke with blade" , "Might one argue" , "Oarswoman or oarsman" , "He handles oars" , "Sportsperson" .)