Low point of river, difficult to cross (6)
I believe the answer is:
trough
'low point' is the definition.
'river difficult to cross' is the wordplay.
'river' becomes 'r'.
'difficult' becomes 'tough' (similar in meaning).
'to cross' is an insertion indicator.
'r' placed into 'tough' is 'TROUGH'.
'of' acts as a link.
(Other definitions for trough that I've seen before include "period when little is achieved" , "Pig swill container" , "basin" , "Animal feeding vessel" , "Feed manger" .)