Support dad crossing river (4)
I believe the answer is:
prop
'support' is the definition.
(prop up can mean to support)
'dad crossing river' is the wordplay.
'dad' becomes 'pop' (synonyms).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation).
'pop' placed around 'r' is 'PROP'.
(Other definitions for prop that I've seen before include "Support to keep from falling" , "player involved in scrum" , "A rigid support - 37 Across is one" , "A rigid supprot or piece of satge furniture (4)" , "One is usually among the strongest and heaviest rugby players" .)