Pulling barrow over river, discovered that saves time (8)

'pulling' is the definition.
(thesaurus)

'barrow over river discovered that saves time' is the wordplay.
'barrow' becomes 'trac' (I can't explain this - if you can you should believe this answer much more).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'discovered' means to look at the middle letters (dis-covered - outer letters removed).
'that saves' indicates putting letters inside.
'time' becomes 't' (abbreviation).
The central letters of 'river' are 'ive'.
'trac'+'ive'='tracive'
'tracive' going around 't' is 'TRACTIVE'.

Can you help me to learn more?