Toll to cross river? No charge (4)
I believe the answer is:
free
'no charge' is the definition.
(synonyms)
'toll to cross river?' is the wordplay.
'toll' becomes 'fee' (I've seen this in another clue).
'to cross' is an insertion indicator (some letters cross or go round others).
'river?' becomes 'r' (abbreviation for river).
'fee' enclosing 'r' is 'FREE'.
(Other definitions for free that I've seen before include "Unconfined" , "Sewn-on trimming" , "See 8" , "nought" , "Untie" .)