Season when tot is with me on river (6)
I believe the answer is:
summer
'season' is the definition.
(summer is a season)
'tot is with me on river' is the wordplay.
'tot' becomes 'sum' (synonyms).
'is with' is a charade indicator (letters next to each other).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'river' becomes 'r' (abbreviation).
'sum'+'me'+'r'='SUMMER'
'when' is the link.
(Other definitions for summer that I've seen before include "Adder" , "warm spell" , "Best of times" , "Warm season, allegedly" , "'Shall I compare thee to a . . . . . .'s day?' (Shakespeare)" .)