Clip youngster crossing river (4)
I believe the answer is:
trot
'clip' is the definition.
('trot' can be a synonym of 'clip')
'youngster crossing river' is the wordplay.
'youngster' becomes 'tot' (tot is a kind of youngster**).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r'.
'tot' going around 'r' is 'TROT'.
(Other definitions for trot that I've seen before include "Horse's gait faster than a walk" , "Old communist" , "Go or walk briskly" , "Hurry along" , "Not dead" .)